Yes we have that ☹️ MIL, she is a lovely woman, we get on well and she is a cherished mum and Nan to our family. But DH younger brother has always been the favourite, he's her baby and she worries about him hugely. When we had our first she said, and this is an exact quote, "don't expect any childcare from me, I'm about to retire and I want to enjoy it" it was said during a big family conversation about how I was going to go back to work after maternity leave. Good for her, no one owes us childcare, thought no more of it.
Fast forward 10 years and BIL has his child, suddenly she's available EVERY DAY to pop round to clean their house, cook them meals, baby sit, had DN for sleepovers from a few weeks old so they could get used to regular sleepovers once mum and dad went back to work because bless them, they'd need their sleep 😂 it does sting a bit. I can see DH is hurt, we haven't said anything yet and still offer her every chance to be fully involved in our lives. That will change the day our children notice the favouritism, then we (but more than likely DH) will absolutely say something.
The thing is she genuinely cannot see it I think, she knows she never babysits for us except in an emergency, we don't ask because I know she's older now and gets more tired. She knows she cancels seeing us and our kids for important things like birthdays because she 'needs' to go over to help them in some way but it never feels malicious, she seems oblivious.
You're not alone, it does hurt and we'll have to say something if our kids mention it, not having them feeling like second best.
Can you have an honest conversation with people? Would she be receptive to the idea she's playing favourite?