How old is the sibling again? If the sibling is now old enough to be in the way and parents are expecting her to change ie not play with certain toys on the floor cause baby will eat them or having to wait too much or whatever there could be a sibling jealousy at the root of it all. Think back to when the sweetness turned sour and what was going on at the time.

The fact it is at home too leads me to believe it has something to do with home. Children that are being molested will even act out this way which is hard because there is a big difference between that and wishing baby sibling would go back to sleeping all day and leave her alone to play which makes getting to the bottom of it all hard.

At 4 you would think she would be saying things verbally too along with the behaviour. Shadowing for sure and calling her out on - I can't trust you so new rules - I move you move simple as that.