I am not sure why we think a MIL has less humility if she is telling/talking about her family.
Maybe, we all need a break, put our moms in our MIL’s shoes. They will for sure love telling their DIL’s (our bhabis how great is our family).
Also, if a bahu or son or daughter or SIL can be moody, why can’t a poor MIL? After all she is a human and has emotions.