We bought a book, "My big girl potty" or something like that. We just looked through the Barnes and Nobles kid section and found it. You just need her to do it a couple times (and praise her to no end) and she'll get the peeing concept (not the trained concept though). We buy a small potty seat to go on the toilet just for them. When I'm first trying to get them to pee in the toilet, I will run the bath water. It helped my son a lot...daughter not so much. Then they have to learn the habit of going to the potty, so I take them every 30 min or so, whether they need to go or not. I try to make it exciting. They don't always think it's exciting. For pooping, I have them sit and ask them to "try to toot!" and they'll try and go poop if they need to.
With baby #2, I would highly suggest just keeping her involved and telling her how exciting it is that a baby is coming. Most kids default to being excited. Treat it as though it's an exciting thing. Then when baby arrives, encourage her to help (but don't force at her young age). My daughter loves to help. It's definitely more complicated when you're trying to take care of a fussy baby, but just keep her close and have her involved however you can. My kids haven't ever seemed jealous of each other. They just love the new babies (#4 is due June).
She'll be entering the more defiant/fit throwing stage, which is just a normal phase. It would happen whether the baby was there or not. It can be a bit complicated sometimes! Anyway, good luck. Having another baby is so exciting. And it's exciting when the baby is around 15-18 months and starts playing with sister. It sounds like you're thoughtful and will figure it out:-) Oh, and we bought a book for bringing home a baby and becoming big sister. Found it in the same kid section as the potty book.