Complex sentence: Members of so-called Bjelbo family became usual holders of jarlship in 11th century and the title was subsumed to duke and powers merged in crown sometime in 1270's.
Split into simpler sentences: Members of so-called Bjelbo family became the usual holders of jarlship in the 11th century. The title was subsumed to duke and the powers were merged into the kingship sometime in 1270's.