If you lose, you'll be imprisoned and your liege will have cause to revoke one of your titles. If you're worried about losing at this point, I would try to get a white peace, which IIRC will prevent this.
Are you certain this is still true? I know it was (and it was this way for a long time), but I recall recently NOT being allowed to imprison/punish the son of a rebelling vassal when the next war ended. It may have been a secondary participant (i.e. co-rebel of a faction, not faction leader), but something struck me as different.