How can my sister separate from her husband

Despite having love marriage, my sister does not feel love from her husband anymore. She feels that he has been ignoring her problems and emotional needs to reestablish connect with him since long time. She feels that he doesn't love her anymore. While he meets all the basic medical needs for her (cluster headache patient, life threatening headaches ) and her daughter (who has type 1 diabetes), he seems to be avoiding lot of reponsibilities by giving seemingly valid excuses because of which my sister is overloaded many times physically and emotionally. He also continues to poke in my sister's matters even when it is not needed. He has the urge to prove that he is right and my sister is wrong and for that he wouldn't care to stretch the discussion to the limits that it causes headache for my sister.My sister does not want to stay with him anymore but does not want to impact father daughter relationship. If she wants to stay in a separate home with her daughter allowing him rights to visit her daughter , is it illegal? If yes, how can she do this legally. Is she bound to pursue divorse to live in this setup. If yes, then can cruelty be her grounds. Could she get child custody