Is a mother's statement to her daughter that the gold in her possession is hers alone, to the exclusion of her siblings—as compensation for not receiving equal gifts as them—considered a valid and enforceable bequest (wasiya) in Islamic law?

The mother is not obliged to give her daughter compensation for what she previously gave her sons, because the daughter was not present at that time. As for the mother's statement to her daughter that her gold is for her after her death, this is a bequest to an heir, which is not permissible unless the heirs agree. The mother must be just in her gifts among her sons and daughters during her lifetime.