Is it permissible to abandon one's sister and limit communication with her to occasions and fulfilling needs, due to her false accusations, especially given that she does not pray and commits other forbidden acts?

Incest is one of the most heinous crimes and a grave sin. If you have repented from this act, you must be wary of Satan and cut off all paths to temptation. Advise your sister and warn her against abandoning prayer. Advise your parents to fear Allah and instruct them to order their daughter to observe the legal hijab and prevent her from mixing with young men. You can seek help from religious tapes, books, and lessons. Boycotting a sister is permissible for a legitimate reason, such as fear of falling into the forbidden or to deter her from evil deeds, and it may become obligatory if it is the only means. However, boycotting for revenge is not permissible for more than three days.