W.T.
Hang in! He's three! :)
I know you mentioned that nothing has changed in your life, but that doesn't mean he doesn't perceive things to have changed. You mentioned that your daughter is getting a lot of attention for "firsts". Remember, he is at the age where he has NO idea of delayed gratification. He cannot rationalize at this age the idea that "she is getting the attention now, but I will get quality time later on with mom and dad". That doesn't work for him. Now now now.
I do think you were perceptive to have picked up on this. I say continue with the rewards, with the reminders to go to the bathroom. He isn't lazy, he is just absorbed in activities. Remember, that a lot of labels become self fulfilling prophecies. I wouldn't "punish" him by taking things away from him. I just think this can delay potty learning and set you guys up for an even longer process.
Involve him in the cleaning up process. Taking soiled clothes to the wash machine, wiping up the floor if there is a mess...not as punishment, just as a natural consequence.
As far as hitting his sister, etc....I don't know if he is too young for a reward system. Maybe every day that he doesn't hit, he gets a sticker on a board. That might be enough for him or you can let him pick out a small toy if he has 3 stickers in a row.
I would simply state the rule to him "no hitting" and redirect. I don't know what you do in your house for consequences. If he enjoys playing with her and hits, maybe he should spend a few minutes playing alone. You could say something like "I see you need some time to yourself" and set him up in a different room doing another activity.
I wouldn't spend too much time talking about being sorry or how it makes his sister feel...kids don't have empathy until about age 6. You can state that hitting hurts, but I would revisit that later during a quiet time, maybe bath time when all is calm and you guys can talk about it.
At the time, state the rule "no hitting" (whatever you choose to say) and have a consequence (redirect, time-out...whatever you think works for your family)...
And again, hang in! He's three!