My mother-in-law absolutely tears my father in law and fiance down verbally, so why do they defend her?

She does nothing but talk **** and throw them under the buss and they are fully aware of it. So why is it that when you try to defend them or make them aware of her hateful condescending behavoir, they get nasty and attack you??? It gets frustrating contantly hearing her do these thing, so why are they so defensive of her??? I don't get it???