As a mother of young adults, I do have some sympathy for your DM, especially as two of my DC have not been in contact with the other for a few months (mainly due to her dick of a BF but that's another story).
One of my DCs says 'you can't force me to have a relationship with X', and I say that I know that I can't and respect their decision. But it is desperately hard as a parent to watch the situation unfold and feel helpless to do anything, especially as they were previously so close. I will be following this thread with interest, as it is helpful for me to see the situation from the siblings' point of view.
I think that you always naturally see your DC as your DC even when they are adults, and that it is understandable that your DM wants healing and reconciliation. I expect she is afraid that she will die and there will be a legacy of family break up. She probably sees it as her fault in some way, even if it isn't.
Would it not be better maybe, as a PP suggested, to initiate some sort of contact/make up with your DB, unless circumstances make you feel that this would not be possible, but I don't get that impression from your post. Life is short and a small gesture might make a big difference, both in terms of getting your DP off your back and preventing further erosion of the family relationships.
I don't think you should regard the situation of them trying to make you 'obey' them, although it sounds as though you have all regressed to parent/child mode; they want you to 'obey' like a good child, and you are being a little pit petulant, saying 'can't, won't'.
Instead, I think you should see yourself as being the mature adult here and taking control of a situation, which is causing upset - for your own peace of mind, if not for your parents.