Tailor Report Link to show Child records from both Parent and Grandparent

I have three tables: a Grandparent table, a Parent table and a Child table. As you might expect, the Grandparent table is in a one-many relationship with the Parent table, and the Parent table is in a one-many relationship with the Child table. As an added complication, the Grandparent table is in its own one-many relationship with the Child table. 

I have a Report Link on the Parent form that I'm using as an embedded report. The behavior I want is to have that embedded report show child records both from the Parent and from that Parent's Grandparent. I do not want to show records from any other Parents under that same Grandparent. 

Is this possible? I've been tinkering with formula fields as the Report Link targets, but I haven't been able to figure out a way to do it.

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Thanks in advance,
Alex
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