Is it considered a sin for a son and his brother to secretly take a portion of their father’s business money to give to their mother, without their father’s knowledge, especially given that the father does not adequately provide for the mother?

If the husband falls short in providing for his wife, it is permissible for her to take from his money what suffices her with fairness, without excess, as was established in the hadith of Hind bint Utbah.

The husband should be advised to be just between his two wives and not to neglect the first wife. It is not advisable to inform her of his second marriage if doing so would lead to harm.