Is the sister obligated to return the financial assistance given to her by her brothers for the marriage of her daughters, especially since they requested her to do so, or to deduct the amount from her inheritance from her father?

The basic principle is that it is not permissible to revoke a gift. However, if the one who spent claims that what he gave was a loan and he intended to reclaim it at the time of spending, then his word is to be accepted, sworn by an oath.