we have two and rotate their duties between housework and taking care of the baby weekly. this was due to the fact that the 'taking care' of the baby tasks was actually much more tiring than actual housework, and one was getting burnt out from it.
we've had them, both quite young, but one 3 years younger than the other. there's definitely a big sister, younger sister thing going on. they've had 2 fights over the past year, we know coz all of the sudden, they are all quiet with each other, but have worked it out eventually. overall, we've been lucky that they've really gotten along well.
due to the rotation in jobs, we do have one 'lazier' helper who always gets a bit less done when it's her week to do housework, leaving the other to 'pick up her pieces' when it's the other week. we notice these but sometimes, it's a bit difficult to correct, but we do try to be fair. in some cases when they have fixed jobs, clear division of tasks is essential.
i agree, it's always a gamble, you almost have to EXPECT that they won't get along and hope for the best.