Is a girl's refusal to marry, whether due to her love for her sister's husband or for psychological reasons, with no legitimate Islamic impediment on her part, considered forbidden in Sharia, and what should she do if that is the case, especially since she is depriving herself of something Allah has made lawful and commanded?

Marriage is subject to the five rulings: obligation, recommendation, prohibition, detestability, and permissibility, depending on the individual's situation. So, for one who fears falling into zina (unlawful sexual intercourse), it becomes obligatory. For one who does not fear zina, it is recommended, unless it leads to something forbidden, in which case it becomes prohibited. For one who has no desire for it, it is detestable if it leads to abandoning something recommended; otherwise, it is permissible. These rulings apply to both men and women, with the addition that for a woman, it becomes obligatory if she is unable to provide for herself and maintain her modesty without marriage. However, marriage is not originally obligatory for women, as evidenced by the Prophet's (peace be upon him) approval of the woman who refrained from marriage.