Questions on first cousin once removed relationship

Hello,

I have a 1st cousin once removed that showed up on 23andme through DNA. I'll call her Sharon. I know who her father is, call him Marvin. I've been trying solve who my father's father is through these DNA links. I'm sure on the surname but I'm trying narrow to the exact person. Marvin fits the surname. Sharon is the closest relative and the best opportunity so far to solve this.

So I'm wondering what conclusions can I make about my father's relationship to Marvin? Does Sharon being a 1st cousin make my father and Marvin brothers in some part? I don't think Marvin is also my father's father or Sharon's relationship would be different and wouldn't show up as a 1st cousin. Is that correct? Could my father's father be a brother of Marvin or maybe Marvin's father or an uncle?

There's also the once removed thing that I don't quite have a grasp on how it might affect the above. If anyone can help with any of this, that would be great.

Thanks.