What is the ruling of the Sharia regarding a father's bequest of one-third (of his estate) to his grandson, while having six daughters (two of whom are unmarried, aged 58 and 53) and two sons, if the motive behind this bequest was to deprive some of the heirs of their full inheritance?

As long as the bequest is within the limits of one-third (of the estate) and is to a non-heir, it is valid even if the heirs do not consent. The testator should not be accused (of ill intent). If he intended to harm the heirs, he would be sinful, and the bequest would be a matter of scholarly disagreement. A bequest exceeding one-third is rejected. If the legatee knows that the testator intended to harm the heirs, it is not permissible for him to accept it. What we deem correct is that the bequest should proceed unless the testator explicitly stated an intention to disinherit the heirs.