When they've been going really well for quite a while and start having accidents, it is often because the novelty has worn off, and they've become a little bit lazy - they don't want to leave play to go and wee *again*

The way to get around this is to make it a real drag to have an accident. So when she has one, get her to the bathroom, where she has to take off all of her own clothes by herself and take them to the washing machine/put in plastic bag/bucket/wherever. Take rsponsibility. Then she has to go and get her clean clothes by herself and then put them on by herself. All of this stuff is hard work, and eventually she'll realise it's easier just to go to the toilet.

When she does go to the toilet, make it as quick and easy as possible and get her back and playing. A strategy to get her to the toilet when she is refusing is to have a special shelf or holding area for whatever she's playing with, so she can come straight back to it. This works for me 98% of the time.

The baby is a possible cause, but don't assume or even think about it. The more you think that's the cause, the more it will be.

Hope this is helpful.