I see your point. I do.
Our situation is a bit different. We do need help. I don't expect the help or for other people to pay our bills or take on our parenting responsibilities. I want grandparents who treat grandchildren equally - doing for one what they do for the other, or at least within the same range.
The situation here is that the other family is pretty well off. My husband's brother is quite successful. Those children, fortunately, are well provided for. Life is never easy street so I don't want to oversimplify their needs, but they are well provided for.
We live below those means. DH makes much less. Most of my money earned goes to daycare and taxes. We live paycheck to paycheck and work a lot to do that.
My kid doesn't have everything he needs. Not wants. Needs. Right now, my child's shoes are pretty worn and have multiple holes in them. It would be nice if Grandpa and Grandma spend so much money on the other grandchildren, if they could spend some on my child's needs too...like shoes.
But it's not their responsibility and I can't make them.
Outside of this forum, I have a full life with career, friends, activities with my child.
If we had a village, I might be more at peace. Raising a child, and holding on to a career is difficult, but even more so without a network or village which is not what MIL and FIL are.
Given your childhood too, I bet it would come up with a bunch of anger and even rage because you've just never had that.
And now you have a son who is probably around the age you can start remembering (fuzzily) the kinds of things you didn't get, and you want to give him ALL of that, and right now you feel like you can only give him SOME of that and here are these people who seem perfectly capable of contributing and yet...they don't, not in the way you need right now.
I think the shoes is a pretty telling point. I'm armchair analysing you a bit which is totally unfair and feel free to ignore, but I'm guessing that being in a position where shoes, a need, are hard to produce is really triggering for you. It does strike me that you're upset with "the other mother" in the family, having had to give up a lot on being angry with your own.
It's hard. And it doesn't really sound all that likely your MIL is going to rise to the occasion, although you could give it one last really brutally open and honest shot like "I know we haven't always been close but our family is totally stressed out and your grandson needs your help and so do I." But I think you would have to be prepared for a no, given the history.
I don't know what their reasons are. I do wonder if it's more to do with their relationship to your husband than to you or your son. I think you've said they told you he's lazy and always has been. That is a really big label to slap on one's child (true or not) and it may indicate that they have just written him off on some level.
I also think you would feel way better if the shoes were resolved. Does your DH have some stuff he could sell on craigslist so you could go get some sweatshop labour cheap but not horribly awful shoes? I realize that was just an example but I think it might be a biggie.