Is it forbidden and does it affect the validity of Hajj not to give the children of a deceased sister a share from the proceeds of selling the furniture of my deceased mother, after giving them their inheritance through the obligatory bequest?

The grandchildren from the daughter's side have no right to their grandmother's inheritance according to Sharia, except through a will not exceeding one-third. The law of compulsory bequest is invalid and contrary to Sharia. Those who have taken based on it must return what they took to the heirs, unless the heirs forgive them. This does not affect the validity of Hajj.