Is it permissible for the aforementioned person to take from his father's inheritance—before its division—what would help him get married, especially since his father had provided his siblings with large sums of money?

The estate must be divided among all heirs according to the legal shares, and what the father spent on some of his children for marriage is not to be deducted, because it is a special expenditure for a need, and he is permitted to do so. Those expenses should not be deducted from those who received them. Sheikh Ibn Uthaymeen said that allocating to the younger children an amount equivalent to what was given to the older ones for marriage is wrong, and the will for it is not enforceable. What the eldest son took from the estate after the father's death must be returned to it to be divided according to the legal shares. The expenses for the children's education have another detailed ruling.