Am I obligated to leave my husband because he committed a sin, "short of adultery," with my niece, knowing that he has repented and ceased the act, and that my family has disowned him and refuses reconciliation? And is it permissible for me to disown my family to pressure them into ending this problem, given my fear of the matter being exposed and its impact on my children?

The husband made a mistake by confessing, and he should have concealed himself. We do not advise the wife to seek a divorce as long as the husband is righteous and has repented. As for boycotting the brothers for the sake of reconciling with the husband, there is no objection to it if a benefit is expected and no greater harm results. Boycotting differs depending on those who boycott and the intention behind it, which is to rebuke the one being boycotted. Its permissibility depends on the prevalence of benefit.