In Girl Genius
Gil is the brother (link taken from wikia) of Zeetha, given the explicit warnings provided by Klaus about someone who looks like Zeetha coming to kill him, the Skifandrian knowledge that Klaus has for staying awake (link), and that Skifandrians hate twins.
In the introduction to "Agatha H. and the Airship City",
Barry heterodyne thinks to himself about the loss of the "Son" of Bill.
Agatha is the daughter of Bill and Lucretia
Who is her brother? Has that been "revealed" yet?