Is the woman who informed her husband of a past transgression committed by his sister considered sinful, and will God forgive her, given that she did not intend defamation and was remorseful?

The default is the obligation to conceal the sins of sinners, especially those who have committed immoral acts, because of the serious consequences of publicizing them. The prohibition of shaming them does not depend on them being harmed. The benefit of your husband protecting himself, so that Allah may protect your children, can be achieved by reminding him of the importance of a Muslim protecting himself without mentioning what happened with his sister. A person's committing an immoral act does not necessitate that his children be punished by falling into similar acts, as Allah the Almighty says: "And every soul earns not [anything] except for itself, and no bearer of burdens will bear the burden of another." You should frequently supplicate for your husband's sister with good, and for her repentance from what she has done by fulfilling its conditions.