I have not used Sandy's, but I have heard enough about them and other ME diapers to reassure you. I used the One-size diaper from the birth of my son, at 7 pounds, with no problem whatsoever. People who use the Sandy's seem to have the same experience. It LOOKS huge on them and it can make some clothes too small, but it works. You just have to get used to the big bum. If the baby is supposed to be on the larger size and your sister is planning on using disposables in the beginning, I would not worry at all about the Small Sandy's fit. But I'm sure other moms with direct experience will give their opinion as well!