I can relate a little here - so I'll tell you our story, and hopefully you can glean something useful out of it!
I started with my son when he was just over 2 - at first, he was SO excited by the potty and I had to coax him back off because he could sit there all day. Problem was, if he did start to pee a little, he'd freak out and want a diaper. But at least he was sitting...
Then he came home one day, and FREAKED out at mention of the potty. And it kept happening. So I moved the potty behind the door where he could see it, but it was not in his face, and we kind of let it go for a while. For the next 6 months or so, I'd ask once in a while if he wanted to potty... nope, nope, nope.
Then suddenly one day - YES! I bought a little treasure chest, and he got to open it after each time he sat on the potty. It varied - a car, stickers, etc. I told him I had a big treasure (a movie) for him if he put poop or pee in the potty. A few days in, he did. Then I only gave him a treasure if he actually did something when he sat (because inevitably, he sat a few times, said he was done and asked for a treasure). We are still at that stage now, but if we are out (and sometimes if I just forget to get it), he doesn't ask for it. The accomplishment has become a reward itself now.
Even though he finally had gone, I noticed that if he had a diaper on, he would just go in it and not even think about it. So in the evenings, I started to put him in a pair of underwear. The first day or 2, he's dribble a little bit before he realized he had to go - but I think actually FEELING that, vs having it absorbed into teh diaper made all the difference. I know all the books and stuff say not to bother with the underwear until they are using the potty already, but for us, that just didn't work... he really needed to be able to feel it to make that connection between the sensation and the need
It's been about a month since we started back in on this process... and Weds we were even at the circus (in a diaper just in case), and he told me he had to potty... so we went. Thurs and Friday he spent the whole day in undies and a diaper in the evening. Each time, he had a small accident around 3:30, that's a time I'm going to have to watch - but otherwise, no problems (although he still hasn't pooped in the potty)
So in my long drawn out way, I guess I am saying that if I hadn't backed off and let him do it when he decided he was ready, that I don't think it would be as easy as it has been. Your little one has a lot going on right now with a new sister and conflicting emotions, so maybe he just needs a little time too.
Good luck
