Is it permissible to stop financially helping a relative who denies the Sunnah, given the inability to advise her and the fear of temptation? And is it better to give charity to Muslims from Ahl al-Sunnah or to relatives who are not from Ahl al-Sunnah?

It is preferable to help people of righteousness and uprightness. It is recommended to allocate charity for the righteous, people of goodness, those of noble character, and those in need. While giving charity to a dissolute person or a disbeliever is permissible and earns reward, it is better to direct charity to those on whom it will be purified, such as the righteous who are obedient to Allah. If there is a benefit in giving charity to the woman mentioned, such as hoping for her return to the truth and softening her heart, then give her charity. Otherwise, it is better to give charity to someone else.