For me, your formulation and especially the title of your post are a bit misleading.
To answer the question suggested by the title, I would answer: in the RFA family document, use a filtered element collector to select all model curve elements and then filter out the one you need from those.
However, when you mention FamilyInstance.get_geometry, you apparently mean something completely different.
I assume you are not trying to access a model line in the RFA, but the family instance geometry in the projects file that results from that line in the family definition.
For that, FamilyInstance.get_geometry is probably the right way to go, and probably also the only way. Maybe it will help to set the Options.IncludeNonVisibleObjects property to true when retrieving the geometry?
https://www.revitapidocs.com/2024/0d277b80-d5e5-1151-d413-b0899da0976f.htm