What should a person do if his parents and sisters commit a sin, and they do not respond to advice, and he fears that they will think he condones their sin? Should he frown at them during the sin and for a short time thereafter?

Enjoining good and forbidding evil is an obligation upon every Muslim according to their ability, and there is no difference in this regard between a relative and a non-relative. A child's enjoining their parents should be done with gentleness and respect, without harshness in speech. If they do not respond, the child should leave them. As for boycotting a sister, it is permissible as a reprimand for her disobedience, and this should be done according to what is most beneficial: if it helps to bring her back to rectitude, then it is preferable; otherwise, maintaining ties with continuous advice is more appropriate.