Is it permissible for a married sister to take a portion of the pension allocated to her widowed sister, who has four children and is in a financially difficult situation?

The retirement pension, if it is from the deceased's accumulated entitlements, is part of his estate and is to be divided among all heirs according to their prescribed shares. Thus, the questioner's married sister has a share in it. However, if it is a grant from the state or the employer, and not from the deceased's entitlements, then it belongs to those specified by the granting authority as a particular category of heirs.