I'm curious why you hesitate to initiate an event to invite the future steps to.  Why do you keep waiting for your dad and FSM to make all the effort?  What's wrong with you having a small gathering before, keeping it casual and low key, pizza perhaps.

Its obvious your dad isn't going to make the effort and I'm sure he never did.  I also wonder why you feel the need to chase him.  In light of that, have you sat down with FSM and had a chat with her about future expectations and pointing out that you would like to keep them a part of your life.  

"Logically, I know he wants a relationship with us but does not know how to go about it even though we all have told him multiple times what we need from him."

Aren't you adults? Your father is no longer obligated to fulfill your "needs".

Here's the reason your father feels more comfortable with FSM's family and not you....they accept him as part of THEIR family. You do not accept FSM as part of YOUR family and make demands on him to see him alone, and advise him that even though you are adults you tell him what he should be doing for YOUR family.

Maybe if you showed your father that you were happy for him and are not as an adult playing "you chose them over us" he might feel more comfortable spending time with you.  Men in general, not just your father, care more about their comfort than anything else. Someone else is making him feel more comfortable than you, quite frankly.  Men also frequently look to the woman to help organize their lives, FSM has shown that she wishes to include you.  If you showed your father you appreciated that and didn't view him as your possession, things might be different.

"He blames us for not having a close relationship but I have tried reaching out to him on numerous occasions and nothing changes."  Has your Father said or done something to give you the impression he blames you?  Could you give a little more information about it?

SacraficalLamb brought up a good point.  Like many men, your Dad probably gravitates to situations where he feels more comfortable.  From what you wrote you are making expectations of your Father that an adult no longer needs.  It sounds as if you are making yourself out to be a victim, in the early stages of forming an emotional triangle.  We still can't determine who the persecutor will be but it's a toss-up between Dad and FSM.

I would suggest you visit a therapist who can help you through all of your feelings and frustrations so you can show your Dad a 'fun' side of you that he wants to be around.  If he still doesn't want to be around you start focusing on yourself and what makes you happy and healthy.

Some men default to their wives/girlfriends for all family and social activities. That could be for all kinds of reasons. Sounds like you know this about him -- as he is just following the social director, your FSM.

While I'm sure you'd rather approach your own father to make plans to see him, your father's reality is that he relies on her for planning. So plan something with your FSM instead of your dad and see what happens. Simple stuff, "FSM, I'd really like to spend some time with you and Dad. Can we get together for dinner next weekend?" If you want a relationship with him, you have to have a relationship with her too.

It doesn't mean he doesn't love you. Please remember that. It means he is somehow socially broken and unable to manage that portion of his life without FSM taking charge.

Sounds like he doesn’t want to make time for you and he’s moving forward with his life. Give him space. He’s planning a wedding! Let him enjoy it. Not everything is about you!

This is a problem that comes up a lot with step-children and step-parents.  Children, tend to think that there is some sort of competition going on between them and the new or future step-parent for daddy’s affection or #1 spot.  There is not.  Being a child and being a spouse or future spouse are two completely different kinds of relationships or roles.  In intact relationships it is more obvious to all that you treat children like children (even adult children) and spouses like spouses.

Not knowing the full past, who can say where the distance with dad and FSM all started?  You made this comment about your brother tho., “He is very against this wedding,” which makes me think something has gone on in the past, something not good.  It sounds to me like there were perhaps at least a couple of events that occurred that made both dad and FSM unsure of her standing or acceptance into the future family/ tribe of her husband’s.

And, from some of your comments I am gleaming at least a little bit that you expect more of a 1:1, toe-to-toe equal relationship with FSM in reference to your roles.  In other words, you are thinking that both you and your FSM are on some sort of level playing field with dad.  Again, you are not.  You are dad’s child (even as an adult) and SM is his wife.  These are COMPLETELY different roles.  Dad loves his children very much, and he loves his wife (SM) very much, but they are different kinds of love.  Men are with their wives 24/7.  Men are with their adult children, much less.   Men expect to grow old with their wives.  Men expect their children to grow and move on and out and create their own lives.

I commend you for realizing that it is up to dad to make the time to see his children (it is!), but maybe this is a classic example of what comes to roost down the road when dad’s GF or future SM is treated like a second-class citizen by dad’s tribe from the start?  Can’t say for sure.  But, men are with their wives 24/7 and want to their wife to be treated like their wife (vs. competition).  Men want to be comfortable and want their wife to be comfortable too.  If anyone (including adult children) make things difficult for either of them, then SM isn’t going to want to go, then dad isn’t going to want to go (not many men want to go to social events sans their spouse) or dad is going to go for only the obligatory hour or two.  Next thing you know, less time with dad = poorer relationship with dad.

Which is what you see going on now here, perhaps.  You and some of your siblings may feel the past is the past, and sure, maybe you didn’t like SM in the beginning, but now you think maybe she is OK or at least you have to try to accept her.  You may be thinking that.  But, both your dad and his wife may be thinking they have had enough and they don’t want to deal with brother or any others in the family who made it clear that SM was not welcomed.  And, now, for their future, dad and his wife (SM) plan to only hang out with people who have no problem whatsoever with them being married to each other.  Can’t say for sure. . .  but maybe it is the chickens from the past finally coming to roost?

If you truly want a better relationship with your dad, I’d suggest that rather than call dad and go behind FSM’s back, so to speak, that you sit down with both of them and apologize.  I know you will probably say you did nothing wrong, but I’m suspecting that this is not entirely true.  Oftentimes siblings, even adult siblings, are in together on family things, including going after or alienating SM.  Now that you finally see it is going to affect your relationship with your dad, suddenly you want to push the reset button.  You may be in that place, but they apparently are not, or at least not yet.  Reconciliation ain’t gonna happen until all are ready to reset.

Observations:  It sounds like to me that you are definitely not happy about your father getting married to the woman he loves. 

The wedding is not and should not be all about you, your brother and sister. Geeze, it's your father's wedding. So, why not just try and be happy for him and his future wife? Your father and fiancé are planning a wedding, so why give him all this baggage before their happy day? Do you really want them to be happy?

Why are you making things out to be like you and your future family are not on the same team?  You mention, "his family" "her family" a lot. Guess what? When you father and fiancée are married, some people consider that being family. So, his wife will be His Family now. 

Being that his future wife and family are going to be part of your father's family, why shouldn't your father spend time with his new extended family on holidays or whenever he wants? It's his life, is it not? My DH is treated a hell of a lot better by his extended family than his selfish, disrespectful adult spawn, so DH enjoys doing things with our family. Yes, it's our family, as we are married and family is not always blood, but family is love. 

After complaining that FSM made plans and didn't invite you-would you or your brother have gone if you felt invited? See, FSM's can never win. Because no matter what they do or don't do, it will never be good enough for adult skids. Seriously, stop trying to control everything and be happy for them and maybe they will be happy to have you around. My guess there is more to the story or it's just that you and your siblings will never be happy for your father that he has found someone he loves.

My adult skids are not happy DH and I are in love and even when invited or not, they just complain and make rude comments. They too feel everything should be about them and the gskids. Suck it up, buttercup. Daddy has a new wife, new life and wants to be happy. My DH was never happy in his marriage and got married because he had to. His friends are so happy for him that he's so happy. 

Hopefully, one day you can all get along with each other.

Imagine if your father threw a huge fit and wasn't happy about you getting married? You don't own your father. Let him move on with his life. He is not beholden to your and your brother. Get a life. Grow up! Annoying.

Lamb described to be absolutely accurate. Your family in some form or fashion indicate an unhappiness or unacceptance of your fathers life which would make anyone gravitate toward and be comfortable with those fully in acceptance of his relationship. Nobody enjoys being uncomfortable.

For you to even think or expect your father to spend Mother’s Day with you because your mother has passed as opposed to his future wife says so much in one statement. Grow up, accept the cards life has dealt you with your mothers passing, and if you love your father as you indicated, be happy for him and stop playing a victim.  Do what you can to make his wife to be and her family feel included. To ask your father not to talk to her about something you want to complain about does NOT do that. You know exactly what you are doing! If you don’t want to lose your remaininf parent, you had better make some changes. I have been exactly in your stepmothers shoes and you and your brother are pushing your father away with your behavior... everybody involved in or looking at his relationship sees it. If you want to keep your father in your life TRUST me you had better make some positive changes SOON. 

This may come across as harsh, however, I have been on the other side of the daughter who has said over and over she has done nothing wrong and only right to be inclusive of her father’s new life as he is accepting and playing the cards as best he can that life has dealt him. Reading your post was like reading some of the ranting notes my husband and I have received from his adult daughter who is never the problem always the victim. I do not mean to be harsh, just real...if you do not make positive changes this part of your life will cause you grief and affect your life and that of your children for many years to come.

I have a dad like this - it's really frustrating. Now I've just let it go... 

But do NOT take the blame for the bad relationship. A lot of guys try to put the emotional labor on women and many get away with it. Well, don't. However, if you want a relationship you know you'll have to do the emotional labor because he isn't willing to do it. Though you do have every right to call him on it. 

And although it's really in your face now, I would put off having the conversation until after the wedding is over. He will be busy and overwhelmed and may take this defensively. But you can say these things at a later date if you want. Relationships don't just have to be on his terms. 

I would also take with a grain of salt all the people who say you're an adult now and so you shouldn't have needs. We all have emotional needs and we all still want a good relationship with our parents - it changes so that they are not always just meeting needs there's a bit of sharing of need-meeting. Obviously your dad still has emotional needs or else he wouldn't complain that the relationship wasn't good. Sometimes though, being an adult, is realising that the people we call parents are unable or unwilling to meet those needs. It sucks. It really sucks. But then we need to do some self-care and stop putting ourselves out for those people. 

For example, I don't like my SM and haven't spoken to her in about 20 years. (I'm not alone in this, her own children contact her only a bit more often.) If my dad wants a relationship he can come and visit me. He does. I mean it's not exactly perfect he hangs out in my house for several weeks (I live in a different country) and basically does not much at all but make mess. But he is who he is. And I'm sure not welcome at her house and I sure don't even want to darken the doorway. You're already in a better position, you do like your SM and don't mind being around her, so it shouldn't be a problem.