It is not as easy to change a Master Source from one Source Template template to another Source Template as it ought to be. And RM's built-in templates themselves cannot be changed. Therefore, one recommendation you will occasionally hear from experienced RM users is not to use RM's built-in templates at all. Even if you don't want to design your own Source Templates, the recommendation is to use copies of RM's built-in Source Templates rather than using the originals because the originals cannot be edited and the copies can be edited. I suspect that virtually nobody follows this recommendation. For example, I do design my own Source Templates rather than using either the originals or copies of the originals. But if you made your own Source Templates, either from scratch or as copies of the built-in Source Templates, you could probably solve your problem by tweaking your templates.
That being said, your problem is the way many of RM's source templates deal with the author field, especially the author field in the Bibliography sentence. In particular, the author should not be reversed in the Footnote sentence nor in the Short Footnote sentence, but should be reversed in the Bibliography sentence. Except that some authors are not a single person. Instead, there can be multiple authors or the author can be be an organization such as a Historical Society or a Genealogical Society. So sometimes the author in a Bibliography sentence should not be reversed after all. Some of RM's Source Templates (I'm thinking particularly of the proliferation of different Source Templates for books) deal with this situation by having one Source Template for an individual author and a different source template for an organization as an author. I avoid this problem in my own templates by having a dummy variable that I can use as a switch in the template and that doesn't even appear in the final sentence.
You didn't mention which of RM's Source Templates you are using. I could provide a little more informed recommendation if I knew which Source Template it was. But in general, you could make a copy of the template you are using, change the copy not to reverse the author's name, and use the copy of the template for a source such as ancestry. Which again raises the question of which template you are using and whether it supports a repository field.
P.S. I wanted to mention also that perhaps ancestry should be treated as a repository rather than as an author. But to tell you the truth, I don't know exactly how to do that in RM short of making your own templates that allow ancestry to be specified in a Repository field. RM's user interface for Master Sources supports a Repository field, but the Repository field is stored in RM's Address table rather in RM's Source Table. There are no Source Template Language variables to fetch the Repository information from the Address table because Source Template Language variables can only fetch from the Source Table.There are a good number of RM's built-in templates that support a Repository field on their own, totally separate from the Repository field that is stored in the Address table. This seems like a curiously awkward design to me, especially since for physical repositories you really do need all the data that's in the Address table.