In general, when younger generation are hosted in older generation's house beyond 3-4 days , then it is good for the DIL& Son duty to take up most of the household chores to relieve the burden on the elderly.Should not behave like entitled guest with own family. If the guest is there for only 3-4 days and want to rest due to long hectic travels/ jet lag etc , then it is justified to avoid household chores like cooking/cleaning or waking up late . But not picking up one's own plate conveys some kind of entitled culture and not due to tiredness.Not picking up one's own plate is very bad manners, no matter who does it. Many times , they trained their son and the FIL to leave the plate for women . That culture has to change first. That said, if your sister is fine with it- that might the the personal dynamic of the family, She may have her own reasons for being extra nice, maybe to compensate for something else in past or present or future . Each family is different. If they are all happy with each other, you should let the matter go- or else you might accidentally end up commenting something to them which will spoil their relations.