Is it obligatory to boycott a father and sister who commit sins and the father does not stop them, and is this boycott considered a severing of kinship ties?

Here is a summary of the answer:

1. Complete your marriage and hasten it to escape the source of temptation in your home.

2. Your relationship with the suitor until the marriage contract is concluded should be strictly limited and only out of necessity, while observing Islamic etiquette. Being alone with him (khalwa) and going out with him is forbidden (haram).

3. Advise your repentant sister gently and wisely, and try to connect her with righteous sisters. If she does not respond, inform your brothers so they can take a firm stance by preventing her from going out unless a mahram is with them, while avoiding violence. Seek a righteous husband for her. It is permissible to prevent her from entering your home after marriage if she persists in her disobedience.

4. Advise your father gently and wisely regarding his abandonment of prayer and slandering chaste women. Avoid being harsh with him. If his harm increases, avoid him to the extent necessary to دفع his harm, while continuing to associate with him kindly.

5. Forgive your sister who insulted your fiancé, because forgiveness increases a Muslim's honor and is among matters requiring strong resolve. It is not permissible to not invite her to the wedding unless the intention is to deter her from disobedience, in order to avoid severing ties of kinship. Be keen on maintaining ties with your relatives unless it results in worldly harm or religious temptation; in such cases, it is permissible to shun them.