... Because I cannot predict my sister's arrival time, she is hardly on time for anything! ...
If you live reasonably close, have your Mom give you a call when Sister arrives. If you don't live close enough for that to be practical, go at a reasonable time and give your Mom as much help cooking, etc., as you have time for before the arrival of Sister and her offspring. Then as soon as dinner is over, remind Sister and Kids that Grandma will need some help clearing the table, doing the dishes and putting things away -- as you're walking out the door to go home.
You can break this unfortunate cycle that has developed throughout your life. But you're the only one who can. Your Mom won't. Your Sister is hardly motivated to do so. The kids don't realize what's going on because it's always
been this way, so to them any other way is unimaginable. But you
can get out the polish and work on your own spine.
If you don't do it soon, it's going to just fester and fester until you explode!