Ah. life life life.

My best friend likes this guy named Nathan, but it kills her because he likes me and not her. I've asked her plenty of times if something ever happened between me and him would it bother her, she says no but I don't believe her because shes like in love...

with the dude but he doesnt want her at all. I feel bad because he likes me and not her, cause shes like my sister. And I like him and want to date him. What should I do?

Mmm, I've been in the same situation as your friend before and my friend went out with him and was really out of order to him (not going into details, but he ended up on anti-depressents etc because of her) so it ruined our friendship completely, but it might not happen like that. Whenever she asked me if it would be ok if something happened I'd tell her it would be fine but I knew it wouldnt really and it wasnt! It may have been ok if she'd have treated him better, so maybe she'll accept it if you dont screw him over and she can see he's happy. Maybe try encouraging her to like other guys or something. As it happens, I got over him pretty quick and now I cant really see what I ever saw in him, so you never know she may just need her eyes opening to another guy (like I did!) and it could turn out fine and eventually just be somehting you can laugh about. Talk to her and tel her that you know she isnt really ok about it, but you do like him and you just want to give it a go, see how it works out, but remind her that you really dont want to hurt her and you're not doing it because of her or anything, just because you and Nathan like each other. And dont whatever you do tell her that she can have him once you've finished with him!!! That's what my friend did, (she was leaving for uni in a couple of months after they started going out, and she told me that she was leaving soon so I could have him once she was done - I WAS NOT IMPRESSED!! lol)
If you do get together with him, make sure that she still cmes number one in your friendship, dont let him take over, and dont blow her off for anything that involves him, make her know that she hasnt lost her best friend as well as the guy she likes. Also when all three of you are together, dont rub it in her face, dont even hold hands if you can help it, that should make things a little easier for her to take.
Sorry that kinda turned into an essay! Hope it helps though!
Feel free to ask more questions!
xox