Do Caroline, Bonnie and Tyler need to be sacrificed as well to break the curse?

Yeah, to break the curse they need a vampire, a werewolf, a witch and doppleganger.
I don't think Bonnie would perform the curse, it would be one of 'The Originals' probably.
As Katherine said, it could of been anyone (like any vampire, any werewolf, any witch) but she chose to play with Elena , if you get what I mean.

Hope I helped?

No, it doesn't NEED to be them it could be aby witch, werewolf,and vampire. She just wasn't ready to sacrafice the Salvatore brothers so she killed Caroline. She also wanted to make Elena suffer considering that she stole Stefan.

that is what Katherine said but i think there's another way. sure she changes caroline and tyler. But bonnie? there is no way, she could have done that. Bonnie will not be killed off because she's a bennett and they are important, (lucy will come back very soon) tyler wont, well i hope he doesnt and caroline i think they could kill her of but would lose so many fans. Anyway they said that they can use the moonstone and they have it too bad stefan locked himself in the tomb in the prosses. So in conclusion, no.

Well, no and yes. A vampire, a werewolf, a doppleganger, a witch, and the moonstone are needed to break the curse. It could be any of them but it deffinetly is most likely that elena is the doppleganger target. Also i dont think the curse will be proformed anyway, soemthing will happen, i know it.