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Little kids who haven't learnt self regulation yet, still are quite reactive over "big feelings" like jealousy, disappointment etc, so this could be a reason why
Jealousy and competitiveness, whats called "sibling rivalry" is very common. Its almost like its a duel to the death to be top dog, and some kids, if they feel threatened by the competence of the brother or sister, will do sneaky damaging things to them or their possessions.
Take the advice above about having the talk, and make them pay restitution for the damage, as that's the only way they will learn they cannot get away with it.
Could be a number of reasons.
A) Feeling angry or jealous of their sibling.
B) Pushing the boundaries, seeing what they can get away with and what the results will be.
C) Lack of empathy - lot of kids struggle with this, most are naturally self-focused, need to be taught early & often to consider other people's feelings.
In any case you should definitely talk to the kid, explain why this isn't ok, and make sure there's a consequence i.e. they should have to replace or repair it, or otherwise do something nice for sibling to make up for it.