Does Allah accept repeated repentance for a sin that has been repeatedly committed?

The Shari'ah has taken precautions by setting boundaries and manners for interaction between non-mahram men and women to close the doors to fitna (temptation/discord), and has warned against leniency in this regard with the husband's relatives. Seclusion with the husband's brother is even more dangerous. Therefore, it is not permissible for a woman to live in the same house with her husband's brother or any other non-mahram man, if facilities are shared or if fitna cannot be guaranteed against.

As for the illicit relationship the questioner fell into with her husband's brother, it is a consequence of her and her husband's negligence regarding Allah's boundaries. She must hasten to repent by ceasing the sin, regretting it, and resolving not to return to it, while increasing good deeds. If she committed zina (fornication/adultery), she has committed a great sin and a grave abomination. Allah the Almighty says: "And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way."

However, sincere repentance is accepted no matter how great the sin, and Allah loves those who repent and transforms their bad deeds into good ones. Part of sincere repentance is avoiding the causes of sin. Therefore, she must demand from her husband an independent dwelling where she can feel safe, and strive in seeking forgiveness and making supplications.