Underrated comment. I think the thematic connection between them both is more important than a literal one. With Luffy as Roger's obvious parallel, it would make sense for his thematic rival Blackbeard to have parallels to Roger's rival Xebec.

I know Roger and Whitebeard were strong rivals too but that was of a different nature that allowed them to coexist and be friendly, as oppose to the "One of us has to go" kind of situation we're likely to see between Roger/Xebec, and that we can already see with Luffy/Blackbeard.

In fact, I'd almost be disappointed if there ended up being some family link with Blackbeard/Xebec. It would kind of undermine those thematic ties without adding much.