Silly might be a bit harsh but you are possibly blowing things out of proportion a little. Firstly, as irriating as it was you friend didn't confirm attendence and perhaps she had forgotten about it until the text. You had a nice time anyway so best not to hold grudges and just bear in mind that for future events this friend may not be the best person to go with if you're booking far in advance.
As to little sister, perhaps she thougth she was doing a good thing. She didn't really want to go but her friends did and she knew how much you wanted to so she had them book you a ticket too. If it's a malicious thing she was doing it's one of the most unmalicious ways I've heard of going about it.
As to your friend now choosing to book a ticket, well I'm a bit lost there. I don't really see how that has anything to do with you. If they want to go then that's up to them I would have thought.