Should I remain silent and distance myself from my sister, as I discovered she is having an online affair with a man other than her husband, or will I bear a sin for leaving her in this state?

A woman establishing a relationship with a foreign man is a clear evil and a manifest sin. Its ugliness increases if the woman is married, because by doing so, she disobeys her Lord and betrays her husband, whom she was commanded to safeguard in herself and his property. Allah Almighty says: "So righteous women are devoutly obedient, guarding in [the husband's] absence what Allah would have them guard."

One must continue to advise the sister gently and kindly, and warn her of Allah and His punishment. If she persists, she should be threatened with someone who can deter her, such as her guardian. If she does not desist, then whoever can deter her must be informed, and if necessary, her husband should be informed to prevent the problem from escalating before it is too late.