I too believe you sister had a scent on her for your snake to latch on. Usually they will bite (even when hungry) but realize by the scent that a thumb is not a prey item. Hard to tell, from here. I was looking away when it happened and didn't see it for myself.

Don't worry about getting tagged. It will happen again, probably more than once. It's a part of the job that kind of sucks!

Jim Smith