M.G.
I come from a family where we all are very direct and to the point. If I need help, I call and ask for it.
My husband's family is the exact opposite. No one is direct, no one is clear, you're supposed to be a mind reader, and everyone's feelings get hurt inadvertently.
I realized long ago that I have to accept them as they are, and lower my expectations - because they weren't going to be helpful (not like my family is). They only want to do the grandparent thing when it suits them. So I approach them like this - "What would suit you?" or "What would work for you?". It's all how you approach it. Once they tell me how they want to be involved, I hold them to it.
So could you go to your mother and ask her what would suit her? Forget what she's done in the past and just go forward and say "Mom, is there a day you'd like to visit with your grandson?" Come up with an arrangement she is comfortable with. Then hold her to it.
Make sure your child is available those days and I wouldn't discuss the time he spends with your uncle.
If your mom won't commit, then at least you know she is not interested. You might not know why, but you have to let that part of it go. Concentrate on yourself and your kids. You invite her to be a part of yours/their lives, and if she agrees, you hold her to what she's agreed to.
I was able to work with my in-laws like this. Whenever it's a loose arrangement and not specific, it's terrible. This way, I feel we arrived at a plan together, and I know exactly what to expect as do they.
Hope that helps :) Good luck