I'd start off with an acoustic. If your sister's has a broken string then what's keeping you from buying a new set of strings? Red Label strings are good enough for beginners and tend to last a bit if you're careful about not overtuning. If you really want an electric, they do make acoustic-electric violins as well.

And I concur with having someone experienced show you how to tune first, as it can be tedious on a violin especially if the pegs slip a lot (which can be remedied using peg drops). If you don't have a good ear, invest in a tuner or use This Site