You have to show.
If you don't muck before showdown (IE you check or call the river), they have a right to see your cards. Etiquette says that it's impolite to ask you to show your cards if you decide to muck, but they *do* have a right to see them if they demand it. Basically this is to prevent collusion. If you demand to see their cards, it's like accusing someone of collusion.
HOWEVER in this case two people were all in, so they the both have the right to see your cards and it's actually improper of you to not show right away. In any online game you play, those cards are shown face up on the table (no option to muck).
Roberts Rules of Poker. Section 15 - Tournaments
34. All hands will be turned faceup whenever a player is all-in and betting action is complete.