When replying to the sender of email sent also to other people (multiple email addresses in CC field) should I reply to all or just to the sender? I think it makes more sense top reply only to the sender.
If the people in the CC are people and not company wide email distribution lists*, then reply to all. The first person wanted them informed, so you should keep them informed by replying to all.
*We've probably all seen the email threads where it ends up being dozens of people saying to take them off the email. Don't include company wide DLs unless your company is twenty people.