this girl spends all her time with me but has a boyfriend, I'm starting to like her

Hi
So there's this girl that I became friends with like 2 months ago, but she already had a boyfriend at the time, but he's a year older and has therefore left the school (we have seen each other but only said hi so I don't really know him)

So over the 2 months, we started getting really close and we are ALWAYS found together, and everyone is seeing that. Friends jokingly tell me to ask her to prom and whatnot, and someone asked me if I'm with her (aka if she's my gf) and he was serious.

One time we went to an ice cream place alone and it was like a date (but not really), and we shared stuff like sexual fantasies etc. (But we were laughing around about it). She also said that before her and her bf got together, she used to ask him stuff like that
Shes also told me they have sex like twice a week before

And when we were there, she asked me to rate her on personality from 1-10 and I said "high enough" then asked her to rate me, she said "9 because you're mean to me" (I'm usually jokingly mean to her), so she asked me to actually rate her now, and I gave her a 10, then she was like "oh that's so nice, fine I'll give you back the 1"

INTERESTINGLY, she's asked me to borrow my sweater that I have in my locker (but don't use) several times, which everyone found weird because I'm like being her boyfrind
However she often refers to me as bro/broski and tells me how were gonna be family friends

She also tells me that she always establishes herself as a bro with the guys

So what's going on here? Please help!!
Thanks!