Hi,

My advice would have been that you should tell your mother about your observations without suggesting anything (i.e. tell her that he has not been home a lot for unknown reason). It looks like you did that already.

You shouldn't feel like a fool at all! She is your mother, it is only natural to care for her and her safety! Not caring about this would have been worse.

Your brother's explanation doesn't fit so well to your observations. Why is your stepfather working late and taking business trips only when your mother is away? And why didn't he talk to you himself? I think you have all reasons to be suspicious.