K.H.
I would say definitely do not give your sister advice. Think of the last time someone tried to "correct" your parenting. It does not go well.
You might also try the book "How Much is Enough?" by Jean Illsey Clarke or the book "Living Simply with Children" by Marie Sherlock. I'd recommend reading them yourself first. You could leave them strategically out at your house and see if she picks them up - but giving them to her as a "gift" will be seen as the comment on her parenting that it is.
Since the children are not being abused or otherwise mistreated, I think your best bet is to wait until your sister tells you she's becoming concerned ("We have so much stuff! Cleaning it all is driving me crazy!") ("I think my kids have too much stuff - every holiday they get so greedy!") If she never brings it up, I don't think you should.