Am i in love with bella?

I realize that people say when your a teenager you don't know what love is. well i recently met a girl named isabella. I couldnt spend much time with her because i had to get to bed because i had a golf tournament in the morning. My brother spent the whole night with her and i wish i could have been there. I met her again camping a different night and she seemed kinda flirty but she knows my younger brother better than me. It also seems kinda wierd because shi is 13 and i am 15 so i dont want to seem like a creep or something. Well for the past week she pops into my head every 5 minutes and i constantly am thinking about her. I really dont know what i should do. ALSO, she is my dads girlfriends neice. should i leave her alone because she is so young and just try to move on. or should i try to get together with her? I am a confused young man. P.S. my brother likes her too and he is 11. It seems like any nice cute girl i meet is either too old or too young for me. wtf?!?