Hi N.,
First since you will be having a new baby in the house here in a few months I would make the transition from diapers to pull ups...babies wear diapers and big girls wear pull ups. You can also start dabbling in potty training for instance take her to the potty at key areas during the day when she wakes up in the morning, after nap, before bath, and before bed, I would start with those areas first.
I would save the majority of the potty training for after the baby gets here and when the house is settled back down. So like when your new baby is about the 3 month mark. My youngest one was born just before my older daughter turned 3. She was potty trained 2 months before her 3rd birthday. We went cold turkey from pull ups to panties.
Here's an example for you.... One of my daycare girls will be 3 in June, her little brother is expected to arrive somewhere around mid-Feburary. Her mother has taken her to the potty since she was 18 months old. So now she can tell you when she has pooped, sge can bring you a pull up and wipes, she can even pee on the potty when she has to go and stay dry all day in panties, she has all the classic signs that she is ready to be potty trained. So 2 months ago we tried she stayed in panties the whole week and dry all day even thru nap but she wouldn't poop on the potty. She went 4 days without pooping and went for 2 weeks like that. When she has to poop if you set her on the potty everything "locks" up so her mother and I agree that it will be better to wait until after she has her brother and we wait until he has been home for awhile.
For me I waited altogether until my 4th was about 4 months old and then pottytrained my e yr old. The first week, first day we had 3 accidents the 2nd day we had and the 3rd we had 1, the rest of the week we didn't have any. The following week we had one accident on a Wednesday and since then we haven't had any, not even through the night.
So what is my opinion ....WAIT. Have you ever heard the term slower in faster well that applies to this case. Every child is different but no matter the child or the parent there will be a transition period when you bring a new sibling into the house not just for the child but for the parents also. No reason to cause more stress and there isn'tany more needed. JMO though, W.