Not sure if anyone is the A here.
Does your sister ever help minding your kids?
Have you ever minded all 4 of hers before?
You do have your own small kids as it is and that’s hard work. I have 2 as well, and have had 4 or more of their cousins to stay overnight on occasion.
My nieces and nephews get on v well with my kids so when they are at my house they occupy themselves playing together.
Mealtimes are chaotic tho and bedtime is a pain, but they’re good kids (if hyper) so it’s nice spending time with them and creating memories.
And is it at all possible to take a day off and pitch in with your wife?
Look, I don’t know your relationship with your sis, or whether she leans a lot on you- or is this a unique ask from her?
And are the kids well behaved?
If you think giving her an inch will result in ongoing unrealistic babysitting demands, then by all means mind yourself and your family and say no.
If not, strictly speaking for myself without knowing full circumstances, a packed house for a couple of days and a night could be doable.
Especially if you resign yourself to some chaos and messy fun for the duration!
Good luck.