When I was working with our daughter on potty training, I let her lead the way. What worked for us was creating a potty chart for stickers and giving her a larger reward when she went in the potty 6 times. She really enjoys dress-up so I printed out a picture of each reward she would receive and hung it next to the chart, starting with a Snow White dress. Each reward after the dress went with it to make an entire outfit (the crown, shoes, wand). Once she went six times, and got the dress, she really wanted to continue going. By the time she had the entire outfit she was 100% trained. It was pretty messy the first week, but I kept her in panties for the entire process. We never made a big deal out of the potty itself, just the fact that she went. I was nursing our 6 month old at the time (and found out I was pregnant with our third). Try setting the timer and taking her to the potty every 10-15 min. and remind her of what she gets when she goes six times. This will help her start to sense when she needs to go and will also help you with making sure she's had a chance to go before you have to nurse again. (It never fails, she will ask you to go potty 3 minutes after you sit down and start nursing). One other thing I did was keep a spare potty in the back of the car. When she would let me know she had to go, she meant NOW. With having two kids, I didn't have time to get them out of the car and into a store before she wet her pants. So when she said she had to go I would pull over, leave the baby in his seat and take her to go potty. You can put a plastic bag in the bowl of the potty and throw it out, waste and all, instead of driving around with a full potty.
I don't think that she is resistent because of your other daughter. I would bet that she was excited about doing something on her own (she's a big girl compared to her baby sister) and you probably made such a big deal out of it with the potty party that she doesn't feel it's her "thing" anymore.
Good luck!