Can we talk about this line? Because I’m never one to just leave a comment be, joke or not, and have been wondering about this for years. Some potential options:
Some branch of the Pratt family, which means that Spike actually pays enough attention to his genealogy to know specifics. Who is this cousin? Does he actually know them? What’s his family tree looking like now, and how mingled are they now within the supernatural world?
A branch of the Aurelius family, so not so much a “cousin” as another fourth or so generation vampire in the line. Who is this vampire, and who sired them? What’s a demon wedding even like? Again, this is a weirdly specific detail for him to know since he's not directly involved in the Order and doesn’t seem to care about any vamps outside the Fanged Four.
There is no cousin. It’s a joke entirely on the merits of winding Buffy up. It could be any random someone or a complete fiction. Which is a reality I reject. Where is your sense of whimsy??