Spotty DD went through a phase of this a while ago, it lasted for about a month. Every time she didn't get her own way, a few minutes later she would have an "accident".
We would send her to her room, and make it quite clear that since she hadn't even tried to get to a toilet then it wasn't an accident. So if she was just a bit late getting there we would reassure her. But it was obvious the link to not getting her own way.
It gradually stopped of its own accord.
I don't know what else to suggest, but agree that the fact she followed it up by pushing her brother over says she was after a reaction from you.
Is there anything worrying her? Or is she just going through a phase?!
A is a cunning little so-and-so. At home he doesn't want to feed himself very often (and then only with his hands) and if we leave him alone with a plate or bowl of food then it rapidly gets emptied on the floor so he can bang the plate on the table/tray. Even strapped into his highchair he'll wriggle and try to get away.
Yesterday DH did the nursery drop off and had to go back into the room where the babies/toddlers were having breakfast about 5 mins after he'd left A there. He found him sitting still at the table on a proper (little) chair, feeding himself cereal with a spoon. Apparently he looked guilty when he realised DH was watching.