Is there sin upon a girl if she leaves her mother alone to return to her studies and continue her life, in light of her family's opposition to her decision to wear the niqab and her unwillingness to work, and their attempts to malign her fiancé?

If your travel for study is safe, then there is no blame upon you, and you must strive to convince your mother.

Neither your mother nor your sister has the right to ask you to remove your niqab.

If the young man is religious and of good character, and your mother refuses to complete your marriage to him, then we advise you to try and convince her and to involve people of goodwill. If she is not convinced, you can refer the matter to the authorities responsible for personal status issues.

The maintenance of a daughter who has no wealth is obligatory upon the father until she marries.

If your mother did not take from your money out of need, then you have the right to demand that money back from her.