What is going through my little sister's head?

Ok so I've known this girl since she was 2. We are neighbors and I don't consider us friends, more like sisters. Now she is 13 and I'm 15. She's in 7th grade and I'm in 10th. I completely act like her older sister but I still always keep her feelings into consideration. So a few months ago I became great friends with this guy about 6th months younger than me. He asked me out, I wasn't interested in him right then so I said no. Now that the new year has started he got held back and he's in 8th grade in the same school with my little sister. They met and became friends, and neither of them knew about the other one knowing me. Then my sister told me abut this guy and I told her how that's the one who asked me out. I asked her if she liked him and she said no. Then now a few months ped and my real sister who is actually related to me met him and the boy knew how that was my sister but my sister didn't know the history of this boy and me. I told my sister about him and she told this boy to ask me out again! So this boy texted me and said, "let's get to know each other then maybe in a month or two you can be my girlfriend. what do you think?" I said, "well i kinda think (my little sister who really is my friend) might like you so I'll have to talk about it with her, because I don't want her to get angry over this." he said ok, i talked to my friend and she was totally against the idea. She was saying, "well you said you thought he was a flirt and a player, I don't think you should." then i told her how all i wanted was a boyfriend and she can get whoever she wants. Then she said that wasn't true and how no guys would ever go out with her. I told her that was bs, and she knows it. Plus he's more in my age range than hers. Then she said "i don't think you should." then a few hours later she texted me and said, "you can go out with him, i don't care." then he Signature said "I LOVE ZACH!!!!!!!!!!" I asked her what that was about and she said that was her boy Friend....so she must have asked him out right after our conversation about me going out with a guy she likes. I don't know what to do..??