What is the ruling on revealing the secret of the husband's sister who is involved in a grave sin, given the fear of her falling into an even greater sin, and does fulfilling a promise take precedence over preventing evil?

Deal with the problem wisely and rationally, and utilize the trust the girl has placed in you by advising and guiding her, and by warning her of the consequences of sin in this world and the hereafter. Inform her that persistence in sin makes it a grave matter.

If the girl does not desist from her sin after advice and warning, you may threaten her with informing her brother about the matter. If she repents and her condition improves, then concealment is preferable.

However, if she does not desist after that, you are not obligated to conceal her; because her persistence in sin will lead to harm for herself and others, and there is no blame upon you for breaking your promise of concealment; because fulfilling it leads to great evils.

In summary: If the girl accepts advice after being advised, then that is good and excellent. But if she does not accept advice, or if you are unable to advise her, then it is obligatory to inform her brother for her own benefit.