At 16 I wouldn't be letting them sleep together. But that's because of historical reasons with showing a lack of consideration for ther est of the family.
And I certainly wouldn't be letting the g/f live in like this. She needs to get a life, smitten as she might be with DSS she shoudn't just be waiting on him. Perhaps she could go home and help her own mum out?
Step mum has nothing to do with it.
This is your house, you get to decide what goes on there. You're not interfering with your DSS's relationship or his life, you're just asking for your home back. Unless DP is actively asking for her to stay over all the time, he can have no argument about you preferrnig she went home.
DSS might get a bit chippy about having to go see g/f or have to walk her home at night but... tough!
We had a similar issue with our daughter. She just seemed to be spending all her time at b/f's house whether he was there or not. His parents were useless at any sort of curfew (she was still at school) and she often came home looking awful and pale and starving!
In the end we said enough as enough, she could stay over at weekends and that's worked really well. She's much happier in herself and much less dependent on him.