It would be easier if people included the full story and not a partial one; it saves time and energy.
Op, you need to talk to your husband about his behavior if you haven't already done so. And if your cousin (sister's fiance) is also mocking your sister behind her back, then I think she should let this rishta go. What your husband is doing is more than just gossip, it's fitnah. Seriously, it's fitnah to turn people against one another and he needs to know this. If your sister and cousin were married, your husband's actions would be even worse as in nothing is said to please shaitan more than to cause separation between husband and wife.
Also, when you say that your cousin told this to your sister....is this cousin your sister's fiance? If so, how is he with your sister? Does he respect her in person and in her absence?
It sounds like two guys joking around about married life...your husband (even though he sounds like an jerk) didn't say anything mean in that exchange to your potential BIL. *
Now regarding your sister: your potential BIL is marrying her because he likes her I am assuming? *Is this a love marriage? *
There's really no advice to give because you seem to be giving us bits and pieces here and there whenever you think we're not telling you what you want to hear.
If your husband is filling your cousin's ear against your sister then could it be the motive here that your husband wants your sister to be married to one of his disabled brothers?
