Back off right now and wait and see what happens. If your sister starts to reject him again (and I mean... if they don't go out at all but keep playing games), then go for it.
Lots of guys in the world, only a limited amount of sisters. Run from him.
I don't know how old you and your sister are, but I cannot imagine damaging your relationship with your sister over a potential boyfriend. She's your sister forever.
Number one: never fish in the same pond as family members, best friends or co-workers. Number two: seems that your sister has been playing hard-to-get and is now envious. Make her and yourself aware that in the end it is and was his decision to be with you. I assume that your sister and Rand were not intimate before, so if she rejected him the whole time, you do not really have to feel guilty. Except for number one.