Many co-wives, when they see something that they know is unpleasant in her (in the eyes of their husband) don’t tell her and tend to use this as a bait to draw in her husband towards herself and away from her co-wife. For example if wife 1 has a bad characteristic which their husband doesn’t like, wife 2 (even though she knows about it) will not tell wife 1 words to the effect of, “You have such and such in you (i.e, you always cook rice), our husband doesn’t like this.“
Not giving advice to your co-wife is a defect which stems from low eemaan (faith). The prophet (saw) said:
“The religion is sincerity.” [Bukhaaree & Muslim]
He (saw) also said:
“The rights of the Muslim over his brother are five… when he seeks your advice, advise him…” [Bukhaaree & Muslim]
Many co-wives think that if she does not seek her advice, she is not obliged to give it. This is indeed false, as the Prophet (saw) said:
“The believer is the mirror of his brother, if he sees any fault in him, he corrects it.” [Al-Adab Al-Mufrad]
So co-wives are obliged to help one another out in informing each other of any defects found in them.