Should I ask her out? ?

I'm trying to decide if I should ask this girl out at school tomorrow. I'm pretty sure she likes me because we've been talking for about a month or so now, and my sister is her best friend so she showed me a text from her that said if I asked her out she would say yes. And we were talking about what could happen possibly happen in a while, and she said we could go out. Then she said she doesnt like anyone mire than a friend besides me. So I'm pretty sure she likes me, I'm just not sure about asking her out tomorrow or Monday because she is coming over tomorrow. So should I ask her out tomorrow or Monday, after she comes over this weekend. I figured it would be weird if she said no for some reason, then when she came over it would be akward.