So I'm going from being a single parent of DD who is 17mo to a blended family. My BF has two girls of his own that are 6 and 4. Hr and I coincidentally have the same back story of how our previous marriages ended bc of our spouse cheating. He actually told me his story first and I was sitting there shocked bc of how similar it is. But anyway things between he and I are amazing and I get along with his girls great and he is wonderful with my DD.
He has been throwing around the idea of us moving in, which I'm totally on board with. However, my only concern is all the girls adjusting. They would have to share a room, which his already do but my DD would be the newbie. He has his DDs every other week so they wouldnt be here all the time and i have DD 80% of the time. He has mentioned about eventually getting a bigger house but that won't be happening for at least 2 years bc I just had to short sale the house I had with my XH. Luckily BF bought his house on his own without his X. Any advice on how to make this transition an easy one?
Thanks!

Re: Intro/advice needed
Is this house a two bedroom? So all three girls would share a room?
I would try to find a way to have a play area that is separate from the bedroom (basement, attic, spare office space). You dont want your DD to "take over" the room with toys and things.
If you want to girls to buy in, maybe ask for their ideas. Would they like a loft bed or bunk beds? You might want to invest in a closet system to make the most out of closet space. Give them an opportunity to redecorate - at least buying new bedding or curtains. Since your dd is so little, she is not going to care enough to vote.
Please also realize that while your DD is the one who is there full time, your SDs are the ones who are "giving up" space, so be gracious and appreciative.
As far as your housing situation, My DD is three - 4 in January, and she didnt get territorial until this last year. And only recently. I think you could buy a couple more years before you would need a bigger home.
Take the time to look and find something just right. And don't feel bad about them sharing. Its good character building and temporary.
You will have to organize very methodically to make any shared space work for all the girls, especially when there is a toddler involved. Both for safety reasons and to give the older kids as much of a sense of privacy and ownership of their possessions as possible (since the newcomer will be there more).
My SD and DS shared until they were 6 and 2, respectively. I tried to use shelving units and organization/storage systems that would allow SD's things to be out of DS's reach. Things with small parts that he could swallow went into tubs that locked and only SD had as of that time figured out how to open. Art supplies were kept in a common area out of everyone's reach. And when we had a bunk bed for a while, we looked for one with a ladder we could remove during the day so that DS could not climb it.
The internet has a lot of really good ideas of how to organize a shared shared for multiple children.
Oh and we had a LOT of squabbles and arguments between them while sharing, but it was due to DS developing destructive behaviors and damaging SD's things, nothing to do with being full, half, or step siblings. Do your best to make sure that the older girls have some areas where their stuff will be safe from a meddlesome toddler. Make sure the user fields know that their privacy and ownership is important to you as well as their father and that you want their new step sister to respect them. You want them to feel like you respect them enough to back them up.
Good luck. That's a tight squeeze. Been there.