I saw that one a while back. The amazing thing is that the younger sister is the one with the real voice - she is going to be something in a few years. Not denigrating the older, you understand, but listen closely and the younger has way better control. (In fairness, the older girl may be singing out of her range to match the younger girl's range, hard to say from one video.) I did a little searching when I first stumbled across this and apparently they actually play musical sisters on some TV program or something.
John