Dear OP, I can understand you. You are a beautiful soul. I understand that on one side you know what you went through and wouldnt want to put your co-sis through same sh!t but you know already and thats why you are mature and didn't involve into the matter. Continue the same. Tell your co-sister that in a newly married life, every girl faces some challenges. Things will not be same as they were around her before marriage. Encourage her to spend her time in bonding with her husband than thinking of her MIL. Let her figure out herself on when to be silent and when to reply back to MIL. Tell her mother to not call you and complain about your MIL. I dont think she should call your husband either. But I understand her mother's plight too. So cant blame her. Be polite and tell her good things about your MIL and tell her that let her daughter get adjusted it takes time. In short advice for you is - "Not my circus, not my monkeys".