I guess the person has two lesbian mothers? Or maybe they're talking about their birth mother and the mother that they raised them. Weird.
I believe the point of this sentence is to show the languages sentence structure and how pluralization works.
Is it common in Westeros (or anywhere else where High Valyrian is used) for an individual to have multiple mothers? (I've never read much of the books or seen much of the show, so I know very little about the cultures in them.)
I don't think this sentence directly correlates with the show. I think it's just a way to expand vocabulary and help us understand and remember the sentence structure.