Should I tell my wife that the rest of her family secretly do not like her mother?

I am pretty tight with my wifes cousin. We were talking and my mother in law came up. and she revealed to me that most of the family does not care for her,they think she is selfish and pushy. My wife has no idea they feel this way about her mother and would be crushed if she knew. I feel like I'm keeping a huge secret but if I tell her it would start a war with an already volatile situation. what should I do?