What is the ruling of Islamic law regarding a woman returning to her first husband, despite her current marriage and having children, and what is the ruling on the ex-husband contacting her brother to ask for her return, and is it permissible to seek a divorce from her current husband because she is not comfortable with him?

It is impermissible for your sister to speak with her ex-husband due to the temptation and betrayal of her current husband that this entails. It is also impermissible for her ex-husband to ask your brother to return her to him, as this would corrupt her relationship with her husband, and it is a major sin. A woman is permitted to seek a khul' (divorce initiated by the wife) from her husband if she dislikes him and is unable to fulfill his rights. However, she is not permitted to seek a khul' without a valid reason, for the Prophet (peace be upon him) counted women who seek khul' without cause among the hypocrites. You both must advise your sister to sever ties with her ex-husband and to be patient with her husband, considering the well-being of her daughter. If their relationship deteriorates, then sensible people should mediate between them.