What to do about a jealous and spoilt three-and-a-half-year-old who hits his 12 month old brother?

My aunt’s son is always hitting the poor baby. He pinches, smacks and pushes him and aggressively gets on top of him and starts hitting him.

When he gets out of hand his mother smacks him but she already spoilt him too much.
What to do about a jealous and spoilt three-and-a-half-year-old who hits his 12 month old brother?
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