In Islam, it is haram to make a will against the rules of Allah. The rules of Allah are to be followed to the letter of the law. However, if the parents feel that there is a circumstance that make their daughters more in need of their money, then they may ask or suggest to their sons to show consideration as regards this case, i.e. to give due regard to their sisters’ condition. There is nothing wrong, as far as Islam is concerned, for the heirs or anyone of them to give his/her share or part of the share to someone else. This will be considered a gift among brothers and sisters.
Moreover, during his life, a man can allocate part of his property to any of his sons or daughters in a form of gift; there is nothing wrong in that, as long as it doesn’t jeopardize the right of others.