Gigis,
You have 2 issues with SIL: cleanup and how she treats her MIL. Irritating on both issues. Please take my notes with a bit of love and thick skin, because telling DSIL off is not going to help anyone, and may relieve you of family you love eventually.
Take a certain soft knowledge with you as you do the practical, appropriate HELP with your MIL on these lunches. You are showing her children the something socially correct for a family: help. You are a great Aunti doing this!
Do not expect her to help, don't expect her to do a thing. Simply help your DMIL instead.
As my Dad would always tell me: worry about your own back yard.
Tennis? Horseback riding? Well, if DMIL wants to be there for her to do these things, maybe she wasn't when she was a girl. And maybe she feels she should be at this point. There isn't a thing you can do.
Her husband? The love of her life? Your husband? The love of your life? She's a primadonna?
Spend some time with her children. You will not regret it.
As far as incapable? She may be. You are not in her skin. She has to learn to be there for her children, and that takes time, as well as emulation. Be thankful she isn't turning hr oldest into a caretaker for the youngers.
You see, I had a grandmother who did just that. She even tried to keep her eldest daughter from getting married, just so she wouldn't have to take care of her youngest (age two at the wedding - and to give you an idea of how things went at the wedding - said 2 year old was drunk on everyone's leftover drinks - hows that for a Mom?), and the olders and the laundry, and the cooking, and the cleaning. She even went to her mother's house and CLEANED and helped out after she was married! Her mother refused to enter our house. Didn't ever once. Would pick her up and drop her off in the driveway.
Incapable? Just might be. Who are we to judge?
Best bet? Go over and help DMIL with DSIL. Then, when she decides she's going to 'escape', kindly but firmly say, no, I have to go home. I have my own house to take care of. Nothing unkind. Nothing mean. Nothing untrue. Give her little tiny bits of LIFE to take a bite out of, and come back and do it all over again.
Two children are OVERWHELMING. Especially. Until you get used to it. If you refuse to get used to it, you're going to be dependent on more and more help. Then it crashes. It burns. And it actually burns the kids when the children are in infancy, so best to help her now if you can.
My pediatrician said it is HARDER to go from one child to two - because when hubby exits that door to go earn the bread and bacon, instead of man on man, you are now on zone defense.
good luck
M.