Is a father's gift of houses, buildings, and lands to his two wives (who are now his widows) acceptable, and do my siblings from the first wife have the right to object to this gift after the father's death, knowing that they had approved during his lifetime?

If the gift was made under its recognized conditions, including the donor's life and health, and the donee's possession and disposal of it, then it is valid, and no one has the right to revoke it.

The children's objection to their father's gift to his wives falls into two categories: 1. Objection regarding lack of fairness in the gift: Their objection is not considered valid, because a husband is not obligated to be fair between his wives in excessive gifts beyond what is obligatory. This is the opinion of the majority of jurists. However, it is preferable to equalize between them to avoid the disagreement of those who consider it obligatory. 2. Objection regarding the validity of the gift: If the gift was not officially documented, the wife claiming the gift (or her heirs) is required to prove it with evidence. Otherwise, the oath is directed to the denying heirs, who swear that they have no knowledge of the gift. If they swear, it is returned to the estate. If they refuse to swear, the wife takes what she claims.

The matter should be referred to the Sharia court in case of disagreement.