Complex sentence: Petersen moved to England in 1966, little knowing that the Bee Gees would soon be doing the same and they recruited him as their permanent drummer shortly afterwards--the first non-Gibb brother to become an official member of the Bee Gees.
Split into simpler sentences: Petersen moved to England in 1966, little knowing that the Bee Gees would soon be doing the same. They recruited him as their permanent drummer shortly afterwards--the first non-Gibb brother to become an official member of the Bee Gees.