Are the divorced woman's family sinful for keeping her four-year-old daughter away from her in order to marry her off, and are they obligated to return the girl to live with them, given that her paternal family does not adhere to religious teachings?

It is necessary to inquire about the ruling on a matter before it occurs. If the child's family knew about the corrupt environment to which they sent their daughter, then they are sinful and must repent and try to bring the child back, because custody takes into account the best interest of the child. However, if they did not know about the corrupt environment, and their motive for giving her away was for the mother's benefit so she could marry with her consent, then there is no blame in that.