ToA p71: "Andreas was the second son of a cousin of Graf Sigismund, patriarch of the very powerful von Jungfreud clan."

ToA p71: "... Andreas von Bruner, Lord Heissman's eldest son..."

ToA p73: " Whilst Lord Heissman is not too concerned with the fact that a wastrel relative has vanished ..."

Chaos at work? Just who is he related to, and why the isolated mention of Graf Sigismund and the von JungFreuds?

Andreas von Bruner, I would say, is the second son of Lord Heissman von Bruner, who is a cousin of Graf Sigismund von Jungfreud.

The relationship being notable simply due to the importance of the von Jungfreuds in the area. Possible friends in high places for the PCs.

You could have him being the eldest son - but I'd say the spare would be treated in such a manner whilst the heir would not - so I'd ignore the mention of him as eldest son in preference to the mention of him as the second son.

That's just my feeling - YMMV.