My Hopes Were Dashed

Okay, I am really going to sound like a novice. I am starting to potty train Mallory. The other night we tempted her to "be a big girl" with brand new undies. She had My Little Pony and Tinkerbell to choose from. She selected which ones she wanted to wear (after trying on 4 of the 6 pairs of underwear) and within 5 minutes she had peed on the couch next to me. Now mind you, she had gone to the bathroom right before the undies were put on. Fortunately it was bath time when her little accident happened. This morning she wanted to go potty when we changed her diaper and guess what?! She pooped all by herself! Yeah, right? She thought she needed undies after we shared a private celebration in the bathroom once I confirmed she did indeed poop. Needless to say my hopes were dashed when not even five minutes later she was standing next to me with a wet skirt and undies. Now I may sound cruel but I know it's going to take a lot more to potty train her. I just thought that maybe I had a gifted child in that department. After all she's watching 3 older sibling go to the bathroom all the time. And what I'd like to know is when the potty break became a family affair?