What is the ruling of Islamic law concerning a wife who became pregnant from an illicit relationship with her husband's brother, and she does not know whether the pregnancy is from her husband or from his brother, and what are the resulting legal rulings regarding the child's lineage?

A husband should satisfy his wife's emotional needs. She should not fall prey to Satan if he fails to do so, especially with the husband's mahrams. One should conceal one's repentance. The child is attributed to your husband, as he is the master of the bed (sahib al-firash), and he is not allowed to disown the child unless there are legal Islamic controls. Adultery does not prevent the continuation of the marriage if the adulteress repents, in order to preserve the family. Divorce is originally prohibited unless there is a need for it. It is advisable to seek wisdom and the help of reasonable people to overcome the problem.