Form Validation for Overlapping Dates

I'm using Planning 11.1.2.3 and have a requirement to implement a validation that checks if level 0 entities have overlapping dates (2 date members that define a range are defined in another dimension). The edit should only check dates among the groups of level 0 siblings; i.e., if there are 100 parents with between 1-10 level 0 siblings each, then first check for overlaps among the first group of siblings, then the next, and so on.

The solution should be dynamic so that no changes will be required if/when the number of parents or siblings change. Any ideas on how to implement this as a form validation in Planning? I know how to build the basic validation logic for determining if 2 date ranges overlap. The difficulty is due to not knowing in advance how many siblings will be in each group and the requirement to limit the comparison to only siblings. The form also displays rows for multiple years, and the comparisons also need to be limited to rows within the same year.