Best friends Lsister likes me

So, we have this little group of friends. 3 guys 2 girls. I'm a guy. One of the girls is my friends little sister, about 2 years younger.

Anyway, she has made it clear she likes me. Apparently has for a long time. She told me she likes some guy and doesn't know what to do. I'm totally oblivious about love and stuff like that. I have never been interesting in anyone. Anyway, I told her that she should just show the guy she likes him, tell him or something like that and that maybe the guy likes her too but is too shy to tell her.

So, some time later she walks up to me and asks me on a date. She didn't call it a date so I didn't realize it at first. We were just eating something. But then she just stared at me and tried to get close to me and soon confessed.

It is so weird. I mean not only she is my friends little sister but she is also my friend and almost like a little sister to me too. To be honest I didn't realize she had grown so much. Now she just keeps trying to get close and I'm worried someone will find out and get the wrong impression. She probably already has somehow. When we were alone in her home she made it clear we were and it would stay that way for a while. She even found a condom somewhere. I quickly sent myself a message saying that I should get home and fast, which she just "happened" so see, so I got out of the situation.

This is just so.... I don't even know what word to use. I can tell I'm definately into girls and not guys, however, I have no interest in kissing or touching or anything like that. I don't even watch porn. I have never really thought about dating or anything like that. I guess she is a nice girl and cute too (it's so weird to write that) but I don't know if I have any interest in her or not. How can one tell? All people at school always think I'm a charming person, almost womanizer because somehow I seem to be popular among guys and girls. I'm just being me and not trying to date anyone but people always get the wrong idea. Anyway, back to the topic.

Should I say something to her? Like.... I have no idea what is going on, I'm confused, please help me. Or maybe just tell her that she is a nice and young girl and I like her but she is like a sister to me. It's so much easier when some random people at school confess.

Should I ask her brother for help? What if he misunderstands? I mean it can't happen but what if it does anyway? What if he thinks I want it and he says it is ok? I'm so overthinking this....

When I say she is like a sister... I don't have siblings so I don't really know what is the difference between sisters and friends (relations). I guess that she used to be so small and helpless and I always had to rescue her out of random situations when her brother wasn't around (or if he was being lazy or "mean" like she says). Actually she is still really small and sometimes helpless lol even though she has grown.