From experience, the best advice I can offer you is to start toilet training when Lily is ready, not when it suits you. We had a dreadful time with Christine because I was convinced that she should start toilet training at 18 months because of the warmer weather, and she just wasn't up for it.
You will most likely notice little changes in her behaviour that indicate she is ready to start moving out of nappies, if you want to encourage her, maybe put the potty in with the toilet and start to talk to her about how 'big people' go.
If you do start toilet training and she acts distressed about it, you might just have to put it off a few weeks and try again. I fully understand your desire to have your toddler out of nappies before the baby comes, but you'll have to guage if it's possible by her behaviour, not by a calendar. I wouldn't wish on anyone the battles we had with Christine, because I was determined that she COULD learn it. I knew better the second time around and let Sam learn at her own pace, when she was ready. They were both well out of nappies before they were two, but the second time around was a lot easier on me!

Kerrie