HELP!

Have you ever been in one of those situations where you REALLY don't know what to do? I AM!

There's this guy at my school. I really hated him last year because he was a real ass to me, but now he's being really nice and I want to be his friend. The problem with that is I have this other friend (just so you don't get confused, i will call the guy Luke and the girl Sarah... those are not their real names obviously) and when I try to be nice to Luke, Sarah get's all pissed at me for some reason. I think she might be jealous, but I don't know. I don't want to lose either of them as a friend, and i don't want to hurt either of thier feelings...
Help?