[SPOILER] Alexander von Brennenburg...

[aquilantiqua]

I'm continually baffled that people don't consider that to be someone's grand uncle (or just uncle) you don't have to be blood related. So say your mother has a sister, and that sister is married to a man. That man is your uncle, but you're not blood related to him right? They don't even have to have children at all, and they're still your aunt and uncle. That man could be an alien insectoid from Neptune, but that doesn't make you part alien insectoid.

So the grand uncle of Mandus could very much be Alexander without him being a partly other-dimensional being, if he just married a German noble woman to keep up appearances while he was masquerading as a baron, and a sibling of that woman is the grandparent of Mandus. Mandus has two coats of arms in his mansion that have heraldic eagles similar to those used in Germany for a long time (as opposed to coats of arms used in England which do not resemble that).

Also Alexander was a founding member of the Order of the Black Eagle and seemingly knew all the Prussian noble families pretty well and they knew him. In general, noble families from an area are all related in some fashion and people, seeing as they thought Alexander was part of the noble line, could have considered him "Uncle Alexander" regardless of just how he was related to them. (Edit: yeah what Alardem said.)