I'm only making suggestions. Unless it's seriously important, it's up to you to decide what to include

You've given us a rather awkward sentence. The grammatical incorrectness makes me wonder what the sentence is actually saying.
It's not grammatically incorrect, only not something a native speaker will ever say. I was trying to be as literal as possible, so as to avoid adding info not present in the original text. A comparatively more natural expression would be "one is married to my grandnephew, the other is married to son of my grandnephew family". The deal with "husband and wife" is that it's just one word 夫婦 in Japanese, and used a lot more often than in English. In English we would just say "my grandnephew family".
If she was already describing her relation to both of them, why would she mention her so i directly, why allow for the ambiguity at all?
Because this sentence has an acceptable level of clarity in Japanese, and ZUN always writes with as much ambiguity as possible. However, as I said, this sentence is only meaningful if it is a reference to the background mythology; it would be strange to present so specific a detail that don't serve any clear purpose.
Today I've been told that Youki being
"Youmu's old man" is a mistranslation. In the context, this sentence has no ambiguity: Youki is Youmu's grandfather. I looked it up in dictionary, and that seems to be true.
BTW, that translation has another important error: Youki believes "skills are learnt by stealing", and cannot be taught by mouth, not "Youki's moves are all stolen".