hiding an affair is one thing
hiding disdain for a family member? I don't know, I guess it could go both ways.
It's not like the girl in the OP was talking "noise" about the dudes family to some Susie nobody co-worker. He was reading her texts that were going to her own sister.
My sister and I share things about people that are less than flattering that we only say because we're siblings, and we grew up together, and we're close.
I mean, 1 reason he's mad is because his fiance complained about his mother's cooking!