Preschooler hitting younger sibling, help!

[DCINightingale]

3.5yo DS has recently become physical towards 1yo DD. He is hitting, kicking, pushing, grabbing hair. I've been trying the respectful parenting garb but he just laughs as I try to stop him hitting her and explain how she is upset, he doesnt seem upset or angry but he must be, it feels like it's just a game as to how much he can get her whilst I try and stop him. Time out becomes just like a tick box exercise for him. I will of course pick her up and ensure her safety. Has anyone any experience or tips on how to effectively handle this?

He can be incredibly sensitive when I correct a small behaviour, so I feel like the laughing is a defense maybe. He can also be incredibly sweet with her, and doesnt display any of this behaviour to anyone else including preschool. It must be sibling jealousy and I'm just finding it really difficult to deal with. I need some help!

Consistent consequence like time out that doesn't give him attention. Fawn over DD when he does it so she gets more attention. But overall, be sure to give ds plenty of attention and 121.

I think at 1 ish the sibling starts to seem more like a child who can pick up things and say words etc, rather than a baby who is a different species and therefore not in competition with the older one.

I found it hard when younger one was at this stage and still very delicate, at 18 months or 2 years he started bashing back and then it became all about refereeing fights rather than fending off dc1 from dc2.