My youngest daughter was a month off 3 before she was toilet trained. We started trying when she was 2, and for 12 weeks she wore "big girl's undies" every day (because she wanted to; after the first 6 weeks I was over the washing lol) yet in those 12 weeks she only peed in the toilet 1x. She was really keen, and demanded to wear the knickers but just never 'got' the concept. We went back to nappies, then just before she turned 3 she did it herself- wanted to wear undies, was told only if she used the toilet, and she did it!!! I tried every incentive I could think of- sticker chart, rewards, (her father used lollies but I didn't!!), everything but it didn't work. Not even wearing wet knickers. She is nearly 4 and still doesn't care if she has an accident...
So basically what I am saying is that they do it in their own time. I felt guilty as an early childhood teacher that I couldn't get it 'right' with my own daughter, but she just wasn't ready and wouldn't do it til SHE was ready...
So good luck, and don't stress too much. While it would be nice to only have 1 in nappies, if you had twins you would have no choice (that's the way I looked at it lol)!!!