There are a number of options, but can't tell enough about the structure of your tables and relationships to know if any of them will work for you. Off hand, I think that changes to the structure of those tables, relationships and your layout design would eliminate the issue altogether by using sub summary layout parts to produce sub headers here.
That said, one way to selectiving indent a field is to put a field to the left of it that's a calculation field that returns different numbers of the tab character based on the value of another field. The visible field would then be set to "slide left" or might be put as a merge field in the same block of layout text as the first field. Note that sliding fields only "slide" in preview mode or when printed/PDF'd.