Maybe he feels he can't challenge his granny and with his brother having a mental health condition it's the same.
That said, he has to have appropriate boundaries for them. Because if my relatives spoke to my husband that way, I'd put them right straight away. In fact even when my BIL (sister's DH), was out of line with my DH, I made it clear I wasn't impressed and would not have gone to their house again (when he was in), if he didn't apologise.
You need to seriously think if this relationship is right for you. I suspect only the thought of loosing you will get the message through to him that he better step up.
In relation to his brother blaming you for him not going to help him... Simply tell him you have no powers to control your DP, and the decision is his own.
Unplug the land line from the wall. The thing is that even if you do that, that's not the main problem.
Don't give up your desire to be a mother, because you don't want your BIL around your DC.
My solution, if my DH can't or won't recognise the issue, would be to end the relationship. If it's making you so upset, what's the point in staying.
You only get one life.