Don’t know what to do

Me and my partner have bought are first house together we have 2 daughters together And my partner has daughter with someone else we get on well I know she’s not my daughter but I think of her has my own she is 17 now it’s been about 4 weeks now since we moved in and iv noticed when her mum is not here she is just wearing underwear and it’s like she wants me to see when she’s been in the shower she’s getting dry and dressed in her room she leaves the door open only last week she stood at her door with her boobs out . Also when me and my partner are having sex iv heard someone outside are door must be her twice this happened are other daughters asleep. And finally what’s disturbing I was going in her room to see if she wants anything for dinner and I heard her saying my name she only had her hand down her nickers really don’t know what to do