What is the optimal course of action regarding my knowledge of my children's paternal aunt having committed a previous illicit relationship, and should I inform my husband (her brother) about it, or should I confront her, and how should I deal with her and with my children's interaction with her, and should I help her get married despite my knowledge of her past, and am I sinful for reading her messages?

You must seek forgiveness and conceal what you have seen in the messages, because concealment is obligatory, and what was mentioned may not be conclusive evidence, even if she has repented from it. However, if a reprehensible act is witnessed, you must advise her and inform her guardian if there is travel without a mahram or if she is left to her own devices. You must also invite her to commitment, influence her, and connect her with righteous companionship. There is no harm in seeking a righteous husband for her, as long as there is no deception involved. As for her interacting with the children, it is permissible if her condition improves, otherwise not.