We used stickers on the calendar for our 3yo son when working on overnight potty-training. WE got one of those huge desk calendars and mounted it on the wall where he would see it every day. Each day he got up dry, he got to put a sticker on the calendar. At the end of the week if he had at least 5 stickers at first (then moved up to 6, 7), he got to pick a movie for movie & popcorn night or go to the park, etc.- usually something free & fun. At the end of the month with stickers on every day except a couple in the first 10 days, we got him something he'd really been wanting. It doesn't have to be expensive, as long as it's something he's excited about & will be motivated by.
Now we're working on chores with the same system (at almost 4, he just has to feed the cats every day). It seems to work great for us!
Just remember that this too shall pass- my brothers (who I took care of) went through behavior issues pretty bad depending on different changes in their lives, but they always passed once things settled down. Maybe her issues resurfacing has to do with the transition from preschool to being home with you & her bro all summer and now to kindergarten..? Best wishes to you & your family.