Mine would be:
Thekla Magdalen
Romaine Maria
I would call their siblings what I would want to name anyone anyway. ;-) But if I did male siblings/twins with thematic middle names they would be:
Karl Egon
Boris Koloman
Or Robert Koloman if I decided to give my offspring something less challenging. The first theme is pretty straightforward but if the second doesn't jump out:
http://en.wikipedia.org/wiki/Egon_Schielehttp://en.wikipedia.org/wiki/Koloman_Moser
Thekla, Romaine, Karl, and Boris
or
Thekla, Romaine, Karl, and Robert