Is it permissible for me to take a portion of my deceased parents' pension, which is rightfully my youngest sister's, to help me with my wedding expenses and other costs, knowing that we used to collectively spend from this pension before I reached the age of 26?

If the pension is part of the father's salary, then it is a right for all heirs. However, if it is a grant from another party, then it belongs to whomever that party designated it for. No one else has the right to share in it, unless the designated recipient, if mature and of sound judgment, agrees to it.