Petrovich - translated as son of Peter. Which really is for sons of Peter and not used like Peterson. So Arthur should be something like Arthur Ectorovich .... <- a suitable dynasty name there, akin to Romanov. Igraine's Russian name is something like Igraine Utherovna (Romanova). It's very patriarchal.
It will be really confusing if you keep them Petroviches.

I'll continue reading now.