As of book 4 we haven't found out much about Cormoran's father, rock star Johnny Rokeby, much less his paternal grandparents. Given Sheila's habit of Looking for Love in All the Wrong Places, pre-Colin, it wouldn't be too much of a stretch that one of their children would inherit the trait, explaining Johnny's multiple children by multiple partners. If Johnny is their first child, born 1964 or so, and if Johnny changed his name to Rokeby and fathered Cormoran in the very late 70s, or early 80's (especially if he was a teenager), the timelines more or less match up. Which would explain why Johnny never denied that he was Cormoran's father, despite Leda Strike's Really Gets Around reputation - anyone seeing a photo of Cormoran next to a photo of Colin would immediately know they're related.
WMG / Poirot