Mud, my two siblings also side with my NM. I expected it from my brother, since he's the GC, but my sister . . . I just don't get it. It almost seems like a form of brainwashing. She's almost 50, yet won't call me or email me because "Mom won't let me talk to you." And as with your situation, my sister has also been abused by M. My NM treats her terribly, yet my S would take a bullet for her.
I talked to my therapist about this, and asked her if she thought they might change after my NM dies, but my therapist felt that no, once Co-Dependent, it's that way forever. After the N dies, the co-spouse or child will continue to abide by her wishes. If my NM has instructed my sister to not have contact with me, it will probably stay that way, as S will be afraid to defy my mother, even in death. Ditto my Co-Father. He removed me from the will on my NM's request, to make her happy because she has terminal cancer. Once she dies, he could easily change the will back, but he won't. These Ns manage to maintain their hold on people from the grave. Amazing, that they have such power and control over others.