Let him follow through on his threat to cancel the wedding.  It sounds like you'd be dodging a bullet.

"Oh say can you see by the dawns early light... that flag was still waving."  Honey.. and it's not red white and blue.. .it's just red. 

Flag 1.  He immediately goes to the nuclear option of cancelling the wedding when you won't agree to what he wants. 

Flag 2.  He wants to move his adult child into the home.  A child that you already indicated does not like you and is liable to be extra difficult to deal with through transition issues.. and you don't feel they will follow house rules.. or agree to any.

Flag 3. He is already dumping his younger kid on you.. "because you are home already".. he is not giving your JOB the respect it deserves. You don't send people to his office for him to hang around with right? Oh yeah.. that kid is trouble too and with an obsession with online games is likely to follow in the older sibling's footsteps.

Flag 4.  You are letting your kid deal with abuse and your DH does nothing to correct his kids.

Look how his other two have turned out?  This guy is NOT a great parent.  He doesn't even seem like that great of a future spouse.  I might take him up on that threat...

Why do you even want to marry him at this point? Your opinions, concerns, wants and needs are being dismissed. 

It won’t get better. Take the out he gave you and cancel the wedding. 

Lord, woman. There are more red flags here than a North Korean parade.  And you are worried about HIM cancelling the wedding?

I'll join the chorus - RUN!

I don't know if DH has always been this flighty or if emotions are just riding hide under the circumstances, but is the 18yo not able to get their own place?  I understand his desire to help TEMPORARILY if BM is being anti-transgender and this young person needs an immediate place to stay, but maybe there's a compromise.  MR. ED and I have agreed that any of our kiddos over 18 are only allowed a VERY TEMPORARY spot on the couch if they ever find themselves in a legitimate homeless situation.  By temporary, I mean like a couple of weeks before finding a place of their own.  Personally, it would take a SERIOUS EMERGENCY situation for me to allow any of my SDs back into this house for one day because they hate me, too.  I feel ya on that one.

That is what happping. Already he dump his 11 yo on you. Now he wants to dump his adult DD.  If BM does not want her,  she must be a pice of work.  Two adult woman can not like in one house.  Let him call off the wedding, then he can take his 11 yo and DD and move out. 

You dodge a really crappy life.  With the SK running everything 

Before you got to the part about HIM canceling the wedding, I was thinking, "CANCEL THE WEDDING!"  This does NOT sound like someone who cares about you or a good situation for you. Why would you want to deal with his kids every day by yourself?  And just "no" to the 18 year-old moving in with no boundaries. This is headed for nothing but misery. PLEASE slow down and give yourself time to reevaluate whether this is the person you want to be with and whether this situation is one that will make you happy, because none of it sounds like a situation that would make me happy!

Girl, there is a lot of wise advice on this board.  Calling off a wedding is not easy.  But your future life as it looks today with this man is gonna be a thousand times harder.

Cancelling a wedding costs less than settling a divorce ...

He and his are already making you miserable - get out while you can and take your life back. You deserve so much better than this.

I would beat him to the punch and gladly call off the wedding. He is showing you who he is and you should believe him.

Younger skid already has issues with respect/integration/ manners etc, now he wants to move in an adult skid with more issues and who also is rude/disrespectful in order to create a totally hostile home environment. Its not a matter of if it will be hostile, IT WILL!!

now he gaslights you to show you are the problem. You should not have a stranger in your home. All the above screams you are not ever gonna be a priority in his life, his kids always will.

he just said the bio mum is gonna be kicking the sd out of the home so since she will be starting mascunilising hormore therapy this will be your problem now.

besides being the babysitter you will be a maid to serve all of them and don’t dare open your mouth to them or you will feel their wrath.

don’t marry this guy, he’s an arsehole for treating you this way

You are brilliant to negotiate how things will go with this skid in the house BEFORE the skid moves in. Stick to your guns on having rules/expectations set in stone before anyone moves anywhere. If he gives you crap for that, or says you can't legislate everything, tell him you're trying to avoid conflict and then ask why he doesn't seem to want to head off conflict before it happens in an already-tense situation. "Don't you want your kids and me to get along?" YES, that's right: put it right back on him the way he's spinning it around to put it on YOU right now.

In the long run, though, it sounds as if he's expecting you to pretty much raise his kids as if they are your own. That's his expectation. Yours is quite different -- you seem to see your future role as a supporting or maybe even something less. This is a vast difference -- if you don't work it out before the wedding with calm discussions and negotiations, you will work it out the hard way after the wedding, with conflict and ugliness.

I can tell you...it's no fun looking back on your first few years of marriage as the worst years in your life. There will be plenty of surprises, especially regarding skids. Even if you must delay the wedding, get this worked out ahead of time. If you want to keep the date, get into serious therapy RIGHT NOW and tell the therapist you'd like help negotiating this specific issue so you and he have a plan BEFORE THE WEDDING.

You must hurry, though. Even after you and he negotiate how you will handle the kids, you will still need time to present that plan to at least the oldest kids, including this skid in question so you're all on the same page. Take the skid to the therapist with you if you must, but be sure he or she knows how things will go after you and your husband agree how they will go -- all  before the wedding.

You aren't talking about leaving the lid off the toothpaste. The issues you and he are having are what divorces are made of. I was really glad I negotiated certain things before we married (I made him PROMISE SD would NEVER live with us no matter what.) I wish I had negotiated more so those first years would be happy memories now.

I’m not against transitioning, but if she is not grown up enough to live on her own and find out who she is as a person, she is not grown up enough to start masculinization treatment. The key to successfull treatment is patience and the right time, and some therapy. People should want to be with her for who she is, and if that’s not enough for them then they aren’t worth being with. This alone makes her a potentially difficult person to live with as she may not listen to reasonable people about reasonable things.  

The above posts are full of good advice, please consider what they are saying. 

OP where did you go?

IF you come back to read the posts, would you please tell us how amazing your boyfriend is? Maybe everyone here has over looked his charm.

Good Luck and best wishes to you!!