I am 17 and I am in love with my best friends sister she is 17 as well how can I tell her ?

To clearify: The BF of your sister loves a 14 year old boy?

You are in pretty cool position - older guys are always more attractive, unless you're gross and he's super sweet... Anyway, don't tell her you LOVE her. Seek an opportunity to flirt with her a little

A party at which she's without the guy? A dance isn't cheatting, right?.
She's at your place and leaves? - "oh, my, I was just leaving too! Let me walk you"

You are talking to her alone somehow ask if she has a boyfriend (unless it's THAT obvious, but if there is a chance you might have not known - ask, show her that you're interested).
And yeah... telling her right away that you fancy her, without her feeling that back or at least realising that you feel this way is a friendship killer...
And one more thing - if you have good relationship with your sister you can ask her about it too - how is it between her and her boyfriend, is she maybe having doubts? Does it bother her he's younger? If yes, then you could show her you are into her. Just ask as if she was free. My brother had a crush on my friend once and him asking me about it wasn't awquard at all.