very important question about Ivan His Booke

[boxofdelights]

"Do you have a lot of cousins?" Tej asked, leaning over his shoulder. "Or just a lot of one cousin?"
He laughed at that last. "Both, actually. On my father's side, there's only my cousin Miles-- not exactly a cousin, our grandmothers were sisters. That part of the family got pretty thinned out during Mad Yuri's War[...]"

Does Ivan not consider Mark his cousin? Why not?