I feel your pain OP but tbh I think YABU, once a fortnight is quite a lot. When my kids were young ,we had my MIL near (FIL had died (they were divorced long ago) before the kids as had my Mum), she was able to help in a limited way which we appreciated, not regularly though and she didn't help me with childcare so I could work, I worked for myself and did it when the kids were in nursery/school or had to take them with me. She actually said to me that if I had gone back to work, I would have gone down in my estimations, she did have some quite strong opinions. . My boys did have a special relationship with her, sadly she died when they were 7 and 5. My Dad is 200 miles away and had taken up with someone new who didn't want to be part of the family. He helped me financially from time to time which I appreciated, but no support with the boys and he hasn't had a relationship with them. Now he is old, his partner has dementia and is in a home. He is living in sheltered accommodation near my Sister, had two 5 week stints of staying with us recently, hard cause my boys wouldn't talk to him. He nearly died four weeks ago (liver aneuryism) and my boys were actually upset. Hoping we are starting to turn things round before its too late.
Difficult, I've done what I can to support him but very hard to let go of resentment, especially when like you I have seen loads of people wih support. TBH I dont know how I'd feel about helping out with Grandchildren regularly myself, firstly will probably still be working cause only have a very small pension, but secondly the idea of looking after young kids again doesn't appeal at all at the moment, I am 46 and perimenopausal though with a teenager and a pre-teen. However, I do expect to feel differently 10 or 15 years down the line and will also feel a moral duty to help out to an extent cause I know what its like to have hardly any help. I might even soften and like young kids again and cope with more mess, who knows. If I was you, I might consider moving closer to your MIL's, but I can see why you would feel bad leaving your Mum.