Any stepparent wins? Advice wanted

Open to constructive advice. I'm (childless 29F) wanting to continue growing a relationship with a 44M with 2 SDs who has 50/50 custody. We've been dating for 5 years and I know the daughters well (short positive interactions throughout those 5 years) but it is without doubt that living with them would change the dynamics between myself, boyfriend and SDs.

I am new to this subreddit so still getting used to the language. Thanks!