Is it permissible for a father to register some of his property to his son who helped in marrying off his sisters, without the sisters' consent? And would the father be sinful if he did that? And is what the father leaves considered an inheritance for all if he dies before registering anything to his son, or does the son take what he spent on his sisters first?

If the brother's action in marrying off his sisters was a voluntary act of benevolence and kindness, then he has no right to demand anything from the father or the sisters. It is also not permissible for the father to favor him with a gift, due to the hadith (definition): "The one who takes back his gift is like a dog that vomits and then returns to its vomit," and the obligation to treat children equally in giving gifts. However, if he spent with the intention of being reimbursed, then the father may give him an amount equal to what he spent, even if the other brothers do not agree, because it is a recompense and a recovery of a right, and not a mere preference.