I'm so sorry. All of these people sound incredibly toxic. Honestly, I don't think you should try to have a relationship with any of them and you should cut ties to protect your mental health however hard it may be. P.S. throw away the whole husband too. After all you have done for him and his rotten kids you deserve way better.

You're super not going to like this.

But ignore her.  All she can gain from this is a response.   Your DH SHOULD talk to his daughter about it, and his son about the gf, and he should talk to his parents about it all, but you should stay away from it.  All that will do is continue to fuel the fire.

I know it sucks, frankly I don't know how you're back after all that.

Sorry you and your child are going through this with this totally disfunctional bunch.

I really agree with Wwcorgi...cut your losses and move on from ALL of this crazy. Both yourself and child deserve better in this life than any of them including your estranged cheating husband.
 

Having a child with someone should not be the reason to stay in a terrible situation that has the very likely potential to damage you and your child. Keep them and all of the crazy out of your lives and be the best parent to your son. I am of the belief that if a child has at least one normal, loving, functional parent they will be fine. However, I do not believe putting yourself and child back in that situation would have a similar outcome.

It may be helpful for you to talk to a therapist and explore why you think being in this situation and family is any good for yourself and child. 
 

If it were me,  I'd walk away from this whole circus.  The skids are unnecessary drama,  and your husband is untrustworthy and unable to control his kids. I think kids are better off being raised by a happy,  stable single parent than exposed to all that.  Don't sacrifice yourself to put your son in a position that's not necessarily better and could be much worse.  There are kids of divorce who do just fine (my skids have handled their parents' divorce well) - it may be the poor interaction  between your H and BM, or just poor parenting by H and BM (not to mention PAS), that caused skids to be a they are. 

I feel so sad reading your story. First of all, don't speak to any of these people ever again. Period. You have no obligation to them whatsoever. Your DH already created a mess with his first family, even if he was largely not to blame for the demise in the first place. You don't mention the circumstances there. Who knows, who cares. Point is, you saw how messed up his kids were, even with your devotion there, and you cared enough to try to make a difference. The foundation for these kids was already laid waaayyyyy before you came along. And the fact that your DH left you to cheat, considering you sound like a decent and caring mother and wife, is pretty clear behavior that he really has no concern for the well being of any of his children, including your bio son. He apparently did not have any inkling of concern about how destroying your relationship with him could affect your son. And this was not his first rodeo. And YOU saw and learned from the previous situation he was in, yet he doesn't? No way. Don't take him back, and don't EVER fool yourself that you cannot raise a well adjusted, successful child on your own. Especially since your son is so young. I have raised my bio, 10, alone all of his life, until recently. I, too, always worried about him growing up with just one parent, even though my situation was totally different than yours. Now that I am married with a SS same age, I see the difference. My kid is not perfect, but I can honestly say that my bio is so much better adjusted all around than my SS. Even better adjusted I'd say than some of my relative's and friend's kids who have grown up in in-tact 2 parent households. Realize that you can and will do a great job raising your son. Find support to do this. And find a healthy male figure to be a constant in your son's life. Be very careful about who you allow in, because not everyone takes child raising seriously. I would not date again until your son is grown, too. If you want to give him a sibling someday, find another route to do so later, adoption, insemination, fostering. Or just be extremely careful about who you choose, preferably someone who is not divorced with kids, but I would wait until you are truly healed from this before making any decisions on this front whatsoever. Just my personal experience. You can raise your son alone, and your son will turn out great, even just with one caring and devoted adult in his life. Even those people who are very strict, and some would say judgmental about divorce and the nuclear family, get behind cheating as an absolute out in a marriage (like in Christianity). Your DH made his bed, now let him lie in it. Trust me, you are not doing a disservice to your child by protecting him from toxic influences, that of which includes your DH. If you haven't divorced him already, by all means do so, and definitely make sure the infidelity is stated as the legal reason. Your son is still young, so you may not get a lot of questions at this point about absent/minimal DH contact. But when you do, never bash your DH, just state the facts of the divorce. Infidelity, period. And get help from an expert on how to do this in age appropriate ways as son develops. When your son is old enough and mature enough, you may just find that he understands and even appreciates the decision making you have done on his behalf at this stage. This is a very crucial time and a difficult time in your life. Be your own best friend right now. This time in your son's life is precious. His foundation is being laid. Do everything within your power to make sure it is a solid one. And if that means cutting toxic people out who have no concern for your son's well being, do it. You are not responsible for their behavior, nor can you change it. 

When you hate his kids, his kids hate you. ( I agree with you about SK) Your DH has no respect you.  Do you want your DS to be in this circus.   Your SK wher not dysfunctional because of the divorce.  They were dysfunctional because of the way they were parent. Or no parenting .  
 

You have a change to raise your DS right.  With out outside dysfunctional influence  on him.  You know SK are jealous of BS. Because he Is getting there share of your money,  your love and gifts. 

Those kids would have been f'd up if their parents stayed together. They are dysfunctional because they are raised by dysfunctional people.  They were dysfunctional before the divorce.  For some people, they use divorce "trauma" as an excuse to continue the damage.

He has alot of toxic baggage.  Its up to you if you can move past him betraying you.  Can he reliability keep all the back stabbing toxicity away from you?

You are not helping your child by going back to a man that cheated on you.  Even if he was perfect, there is too much baggage with the stepkids and their soon to be spouses.  Just wow!  I would think that your child would be more messed up by being around that crew.  Without the peripheral drama, you would still be dealing with trust issues with him.  

Don't think for one second you have to get back with your husband and his dysfunctional family in order to raise your son right. You may actually be doing your some a disservice by getting back together. You seem like a loving devoted parent. That's all your child needs. Introducing him back into the instability and dysfunction you describe will only cause more harm than good. A husband you can't trust. Step siblings and extended family that bad mouth his mother and her character.