How can one strengthen their resolve to stop masturbating and committing sodomy with a relative, especially when exposed to temptation, given its negative impact on prayer performance?

You are in grave danger, and you must hasten to repent to Allah Almighty for the abomination of sodomy, for it is more heinous than adultery. Allah has sent down a punishment for those who commit it that He has not sent down for any other nation. So, beware of Allah's wrath and punishment, and stay away from this person who leads you to abomination. Accompany the good people, frequent the mosques, and increase your seeking of forgiveness and performing good deeds. Masturbation is forbidden, and you must repent from it. Abandoning prayer because of it is a greater sin than committing it.