Yes.

I would question the motivations of the Founder/CEO if they are keeping this from you.

Pre-investment it's slightly different as it's understandable sometimes when negotiating if founders dont want to reveal other potential angels; you can then include that fact in your decision whether to invest. But post investment it would be very unusual and actually unhelpful to the founder/startup itself for investors not to know each other.

Ask for a copy of the CAP table.