Wow, and I thought the women I get involved with are loony. Frankly, I don't see where you did ANYTHING wrong at all, in fact you should be commended for helping your family. What could be more innocent than driving your little Sister someplace? An ex-wife or girlfriend, even perhaps a buddy, yes. But a Sister? No.

Sounds like she has other issues (who knows what) and this was just a lighting rod for them.

What I would do is 'defuse' the situation immediately, hard as it may be don't contact her for at least a few days and then when she calls you (and she will I'll bet) talk calmy and communicate about it.