Is she aware that in a short time there will be a new sibling? What is her temperament towards that situation? Is she excited? If she has some confused feelings about it, maybe right now isn't the time to be trying to potty train.
If she is otherwise ok with it, then I agree with the commenter who suggested you need to figure out the reason she is reluctant to sit on the potty. Neither of mine really cared for "potty chairs." One didn't care for the regular toilet either, but she was FINE if I put a seat attachment on it. And she trained at 2. Actually, she was basically finished training on her 2nd birthday and in "training pants" type underwear already. She never really had accidents, but I was expecting them, so I kept her in those thick waffle weave cotton underpants for a bit, when really, she didn't need them.
My son was trained not long after age 2, and was fully trained before age 3... and did not have ANY regression issues of any kind when his little sister was born 2 weeks before his 3rd birthday. So don't assume that she will regress because of the birth of a sibling. There is more to regression than JUST that. However, since she is new to potty training and the birth is so close, you might be more likely to have issues. My son was an old hand at it when his sister was born, and I suspect that was part of the reason he had no issues. But he didn't regress in other ways, either. Maybe some of that is personality?
We went the route of using candy. Stickers were more of a hassle for me than anything. So I bought lifesavers. Kids want to have some control. If the only thing they can control is using the toilet... well than that is what they will use to exert their control/power. My kids got a "say" in other things. And using the potty afforded them another way to get choices: they got to choose which candy they wanted. The package was mixed flavors, and all the pretty colors were a fun thing for them to choose from. As soon as the deed was done and hands washed... straight to the kitchen to pick a candy from a clear baggie. Quick. Simple. Their power/control/choice.
Eventually, they don't care about the candy... but they are in the habit of being clean/dry and pleasing you. :)
Good luck. And congrats on the one on the way..