Every fan knew. It was a huge scandal, and they stopped playing his records on the radio for years after he married his 13-year-old cousin. At least around Baltimore-DC, that is.
Nah, my mom didn’t know much. And she was Baltimoron.
Of course, she (and I) is not a typical “fan”. She was not into celebrity nonsense like concerts, and fan magazines, or screaming and fainting over them.
Probably more like me - I listen to music, and don’t much care about the details, and just happen to notice when an act is consistently good that we like them. Couldn’t name members of any given group, but I know what their hits are. Just don’t care to know everything about them.