Should the husband be informed about his sister's betrayal of her husband, or should it be concealed?

If it is proven that the wife disobeys Allah and does not guard her husband's honor, then she is a wrongdoer and transgresses the limits of Allah. It is her duty to guard her husband in his absence, as Allah Almighty says: "So righteous women are devoutly obedient, guarding in [the husband's] absence what Allah would have them guard."

If she returns to her bad deeds after returning to her country, then whoever becomes aware of that should advise her and conceal her [faults] if she is not openly defiant. If advising her is not possible, then it is obligatory to inform someone who can deter her, such as her brother, hinting at the issue instead of explicitly stating it, and supplicating for her rectification.