Hi dear, personally i would say it should not be the sister's fault but should be your husband's. Why? Because he should know better, regardless of whoever invited him or went alone! He is a man, a husband and he has a family to take care of yet he chose to go with his sister to a club?! He is absolutely aware of it too.
Talk to him, educate him. He is a functioning adult and you are not going to take care of a big baby. If problem persists, go to couple's councelling.
His priority should be on his family. Your priority should be on yourself and child. Family health is very important for the child's wellbeing. Stay strong, best of luck and take care.
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