Advice?

My bfs niece comes swimming at our house, she's the same age as me he's only 5 years older. And she's hated me ever since we started seeing each other. So today it sucks because it's very hot and I have to work in a few hours and I only have one day off a week but I want to enjoy my few hours before work but she's here so I feel like I can't even be at my own house(technically his bc I just moved in 6 months ago)but still. I think I'm just going to go out there either way but just feel so akwkard cause last time she was here she was being a huge bitch to me. And it's hard to talk to my so about it because I don't want to put him in the middle of it. And he's torn to because that's his niece and I'm his girlfriend. And I don't see why she comes here because her mom has a pool not as big as ours but enough room and she lives a little closer to her than we do. I just hate feeling like I can't be at my house because she's such a bitch to me.