Something tells me that Bo likes his new job enough to not just hire his brother to keep Mom happy. Coaches hire based on three criteria that I can think of: experience, recruiting, and trust. I'm thinking the trust part is a lock. Can't speak to the other two.
Carl is more than a figure head. Bo was a great DC. Now he is concentrating on being a great Head Coach. Bringing in Carl was a no brainer. Why not have someone he trusts completely run the defense. Bo will give Carl more rope than ANYONE else.
Ed, welcome back! I'm a Bo fan, he did a great job here. I hope Carl works out for you guys, I know nothing of his resume. But if it does not work that well, it would be hard for Bo to fire him. Could get sticky. I would not put myself in that situation. But as I say, here's hoping it works out!