We have been struggling with the same question of late. But we honestly have no idea who to choose/ask. I have one brother who has a long term partner and they have four kids of their own. Originally I had thought of them because of the relationship we have with their kids and thought it would be good for DS. However recently some stuff has come to light and there is no way I would choose them now.
Dh has two brothers, one lives at home and the other rents with his girlfriend, but neither of them I believe are suitable.
At this point we don't have a clue.
In your situation I would choose your sister as the kids would still be near their friends and family they know.