K.I.
I think you would be well within your right to tell sister "Hey woman! Come help me with dinner"...and then you can both make the kids' plates (she hers how they like them and you yours), then when its done before anyone gets up from the table say something like "Who wants to clear and who wants to load the dishwasher and who wants to take care of the kids and who wants to clean the counters?"
I get that you have a strained relationship, I do not w/ my sister, so my suggestions might seem strange but you have an opportunity to rebuild that relationship any way you want. If them making messes and not helping with the clean up bothers you so much...talk to your sister! She is a mommy too, and knows full well how hard it is to clean up after others. Just be honest, direct and kind. She will probably understand. If she doesn't, then you will have to reevaluate how much you want to burden yourself in the name of family?
Unless your sister and her family are totally poor and have spent their last dime coming to visit you, and you know it, the request for diapers is ridiculous...her kids, her diapers! I can see having favorite snacks and such but the diapers are over the top...don't be a doormat!
Best of luck to you...I mean what I say about just being honest...it is the best way to go...it is what has kept myself and my sister close all these years. She is my BFF and we talk everyday...most days more than once...and we have both helped each other out when the other needed and HAVE HAD to have the conversation about picking up after each other and each others kids. She will understand!