Lots of things can affect a child’s behaviour. If she feels left out or as if her brother is getting more attention in nappies that may be a reason
change in anything in her life etc

Have a look at her diet. Does she go to Kindy? Have they started to transfer her to another room? Change in Teachers? My son is 2.5 years old, and toilet trained through the day. He’s still having accidents, but, I know that it’s because of the room change. The smallest little thing can be a massive deal to little one’s. Good Luck.