Do the costs of Hajj that I committed to for my father before his death count as a debt transferable to his heirs, or should I give them in charity so that the reward reaches the deceased, and is it permissible to give them to my poor siblings to help them build a house or get married?

If the son agrees with his father that the father will arrange for the son's mother to perform Hajj in exchange for the son paying the father the expenses of Hajj, and the father then performs this, these expenses are considered a debt owed by the son to the father. This debt is then added to the father's estate, like his other assets and debts, and the estate is subsequently divided among all heirs, including the son. This is a matter of consensus among jurists.