Is it permissible to marry the divorced wife of the wife's sister according to Islamic law, with the presence of two witnesses, without the knowledge of her family? And does this marriage require her father's consent, given that her ex-husband threatens her and anyone who marries her with harm?

A legitimate marriage has conditions and pillars without which it is invalid. Among these is the guardian (wali); therefore, it is not permissible to marry off a woman without her guardian (her father, as long as he is competent for guardianship). It is not obligatory to inform all family members, and it is sufficient for two witnesses to attest to the contract.