I'm 16 and he's 19

Me, him and my sister were hanging out. He and my sister were talking about his gf, and what was the first thing he noticed about her, then my sister asked him if he ever had sex with her and he said yes, BUT I STILL WANT YOUR SISTER. I was right there and I'm like wtf (damn I want you too but you are 3 years older than me and you have a gf-I didn't say it out loud though) I don't know what to do and I really do like him. Can you please help the girl with age difference thing...I'm 16 and he's 19.
I'm 16 and he's 19
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