I think she was included to mock the notion of aristocratic bloodlines. Arranged (forced) marriages between royal families were always for the purpose of procreation and, well...

But yeah, you need so much setup and context and certainty around the bona fides of the OP that the punchline itself (if I'm even right about the intended punchline) just gets suffocated.

There's only one guaranteed way this joke would land and that's if Vivian herself had posted it.