What should I do in light of family problems, such as my father abandoning prayer and denying the Sunnah, the ill-manners of my irreligious sister, and my father's prohibition when I try to deter her?

Abandoning prayer is one of the gravest sins. Whoever abandons it out of denial has apostatized from the faith and is to be killed as an infidel; he does not inherit, is not washed, is not prayed over, and is not buried in Muslim cemeteries. As for whoever abandons it out of laziness, he has committed a major disbelief that expels him from the faith, according to the preponderant view of scholars.

Rejecting the Sunnah indicates rejecting the Quran, because the Sunnah clarifies and explains the Quran. Allah the Almighty said concerning the Prophet (peace be upon him): "Nor does he speak from [his own] inclination. It is not but a revelation revealed," and Allah the Almighty said: "And We revealed to you the message that you may make clear to the people what was sent down to them and that they might give thought."

One should strive gently and with noble character to guide family members to adhere to Allah's law, seeking help from those connected to them, and having them hear religious advice and guidance at appropriate times, while resorting to Allah the Almighty in supplicating for them.