NO, you're NOT a failure!! Chances are, BOTH of your girls will be potty trained at the same time, just wait a little bit longer! ;-)
It is my observation that those who potty train earlier often have more accidents (or will be potty trained for pee but not poop). Personally I much prefer to change poopy diapers (unpleasant as they are) than change & clean poopy underpants. Your girl WILL catch on, and your younger one just may be old enough & interested enough to train along??!?
I remember this about my younger sister who was about 3 or 4 at the time (another baby sister younger than her was still in diapers.) My parents friends came over and they had a little girl the same age as my little sister (named Anna, the other girl was named Nana, so we had fun pointing out to them how similar their names were fingerspelled) Anyway, in the course of the evening, the little girl had to go and she went to the potty, leaving the door open at the end of the short hall and I remember seeing my little sister who had come around to find her standing there, staring in surprise. After that, she was potty trained too! It really is a mental thing (once their bodies are also ready). You can battle and train, or you can roll with it and rejoice when it happens.