I have a similar situation, Monrouge. My daughter works three days a week and her MiL is paid to childmind twin grandaughters on those days. A great arrangement - she's a safe pair of hands, 10 years younger than me, and lives a few doors away. If she is ill or unable to have the children, I expect daughter or SiL to cope with this, but I will fill in for emergencies. I'm happy to babysit, but now I have retired I don't want a job that entails charging round after my grandchildren for 8 hours. I want to enjoy them and help in general but not childmind. I go with her and the children to the library and for picnics, as it's difficult to manage twins in some situations, but she offered to childmind and it suits her.
I don't feel shut out - on the contrary, I love to see their trust in her and feel confident that our grandchildren have the best care. I call in to play with them whilst she's looking after them and she knows I am there as back up, if needed.