Is it permissible for sisters to take a sum of money from their sister's frozen pension after her husband's death and her marriage to another, knowing that she is a mother of orphans, and is this money the right of her children only?

If the sister took from her father's pension unjustly, and it cannot be recovered except by taking from her husband's pension, then it is permissible to take only what equals her right from her share of her husband's pension, without affecting the share of her husband's family, and without exceeding what she took. It should be noted that if the pension consists of prior entitlements of the deceased, it is an inheritance to be divided among the heirs according to Sharia (definition). If it is a grant from the state, it is disbursed to whomever the granting authority specifies. The share of minors can only be managed by their guardian, and nothing should be taken from the pension except by those entitled to it or with their permission.