In Al Anon they say not to do anything if the other can do it. How will they ever learn how if we are always there? We can't live forever. Then what?
Where I live they have services for mentally slow people and you can even get rides to the grocery store. Workers will shop with you. This woman does not have to do all her shopping at one time. Telling you, I could not afford that load of groceries!!! But she gets state money. This is a consequence/perk of her having a boat load of kids. As long as you help her, it does not seem as if she is going to stop having babies anytime soon. Have you ever talked to her about having surgery to end her crazy life of bringing more and more kids that she is incapable of raising into the world? Those poor kids.
Are there services for the poor where you live? People paid to help her? Government workers?
Also, you deserve a life too. Are you retired? If so, are there activities that you enjoy? Grandkids are lovely but we all need time for ourselves too.
I can only tell you my own opinion. I am very busy, plan on staying that way, and would not overly give care to kids that were not my grands. I would babysit sometimes. The car rides, not as often as you. I would direct my person to use government services. Now I did not always feel this way. We did EVERYTHING for our daughter. But we see now that this only made her more helpless and also entitled. She never grew up.
In the end, there are no hard and fast rules, just opinions.
Blessings and hugs. You have a good heart.