There are lots of ways to achieve this and it is just a matter of taste. You either set a value on the portal-records, but then as you noticed there emerges the need to change something on all the other records.
Since there is a relation between the "mother" and the "children" you can utilize that relation by sorting the records in the portal on a value in the "children" and that value should be an un-stored calculation, which checks for a value in the "mother".
Create a field in the "mother"-table called "main_id" (or anything else you think is appropriate) and just put the primary-key of the record with the "main-number" there. Create a calc in the "child"-table and check over the relation if the valus set in the "mother" is equal to its primary-key and if so, set 1. Because it is using a relation, it'll be un-stored, but you won't start searches over that field, so it doesn't matter.
Now you can use protal-sorting to show the "main"-record on top of all others in the portal.
SortPortal.fmp12.zip 68.3 K
I would use a script for this. Navigate to the related records, recording the record ID of the record the user clicked. Set that one to "primary". Set all others to "not primary".