Is it permissible to give Zakat al-Mal to a female relative to help her prepare for marriage, and is it considered Zakat al-Mal if the questioner's son intends to marry her, knowing that he will provide for her preparations in both cases?

It is permissible to give a needy daughter from the Zakat money to prepare her for marriage, whether her husband is your son or someone else. It has been reported that an orphan girl should be given from the Zakat what is necessary for her marriage, and what the judge deems appropriate for the minor.