Is it permissible for the first wife to ask for the divorce of the second wife, and do the spouses incur sin if the second wife returns to her husband's wedlock without the first wife's knowledge?

A man marrying more than one woman is permissible in Islamic law, and there is no harm in it to the first wife. It is not permissible for a wife to ask for the divorce of her co-wife, as the Prophet (peace be upon him) forbade that. Therefore, your husband's wife asking for your divorce is forbidden, and your husband should not respond to her. Your husband taking you back during the waiting period is permissible and recommended if the divorce was revocable. We advise both of you to fear Allah and increase in supplication. The matter of reconciliation can be kept secret for fear of harming the first wife, and you should teach your husband that what his first wife is doing is not permissible in Islamic law.