15 yr old stepdaughter

Tiny backstory, 3 months ago my partners daughter 15 came to live with us (i have a 2 yr old girl with partner and 2 sons 9, 12 from previous relationship) he was alienated from his daughter for 5 yrs, first 3 months were perfect, I could not fault her in anything, a switch has now been flicked and I can not stand this girl, I believe it all started when her dad told her to stop walking around the house half naked, I believe she's playing sick mind games with me, insulting me, ignoring me, giving dirty looks, she's making me very uncomfortable in my own home, she's somewhat acting like it's her and her dads home.. i can't explain how but it's just this vibe and intimidation, im isolating myself and sinking into depression, she's coming between me and my kids too, she's trying to parent them, she's constantly telling my sons "tell me you love me" I feel bad for my kids they're uncomfortable with this but I feel helpless I don't know what to say.

I love my partner deeply, we have/had a fantastic loving relationship, I've been telling him what's going on, he's says "if she wants to play mind games she can f##k off back to her mums" but that's not an option, the mum does not want her back, the family friend (mind games played with her too) she lived with does not want her back. Do things change? Is this normal teenage behaviour? My kids need a happy healthy mum, if this is my life now I will leave and start over. Thankyou