My fil is terminally ill and mil has to take care of him as he's very poorly - dh and I help her as much as we can. But her daughter, my sil, still makes quite a lot of demands on her. She and her dh expect mil to come and take care of their children two days a week while she's a work (so fil is left alone all day in the house) and often calls mil at very short notice asking her to come over and look after her children.
A typical example was on Friday - my dh was at mil's house repairing her washing machine for her as fil isn't well enough to do these things anymore. Sil suddenly rang saying she had to go and pick her four year old from preschool but her one year old was asleep and she didn't want to wake her youngest so she asked mil to come over and look after the little one. So mil had to drop everything, drive twenty minutes to babysit and leave fil. Luckily dh was there to keep an eye on him.
It's so hard to watch mil being taken advantage of, we've stood by for a long time and watched it go on but recently as fil has deteriorated, we have gently suggested to mil that maybe she is doing quite a lot for sil but she gets very defensive and sticks up for her and acts like we are the ones being unreasonable. She won't listen to anything remotely negative about sil. Dh gets very upset about it and I must admit I find it staggeringly selfish but if Mil is seemingly happy to do it, I don't know what we can do. I think it's just the dynamics of their relationship - sil is overly dependent on mil and mil has always facilitated this so the pattern is too established. I need my mum lots too but I would like to think that I would know when to back off and sort things out myself. AIBU to be astonished that she can't do this even when her father is so ill.