Parent relief is not meant to compensate taxpayers in full for any particular expense incurred in caring for their parents. It is a form of recognition for individuals taking care of their parents who are 55 years old or more, or are physically or mentally handicapped. Hence, the relief is tied to the particular parent who is being cared for.
Under the current framework, only one sibling can claim relief for each parent. This stipulation is a practical one. Siblings should decide amongst themselves who should claim the parent relief. Allowing apportionment of the fixed quantum of relief among the various siblings based on criteria such as expenses incurred by each sibling is not feasible, as there will be conflicting claims and disputes (given that this situation arises because it is not possible for siblings to decide amongst themselves who should claim the relief and how to share it).