5 year old being mean

[sillyquestionalert]

I don’t know what I’m doing wrong but I’m at my wits end
my 5 year old is a delight at school, no issues at all and a lot of the time he is a sweetheart, so kind and lovely to be around but then on the other hand it’s like he just cant be nice mainly to his older sister, it’s like the mindless automatic say something horrible to her
we have tried what feels like everything and it’s the same circle- he promises to stop but it starts again soon as the conversation is over.
some people say it’s a phase but this can’t carry on, others say he’s probably picking it up from school.
what can I do? What is the best approach for this type of behaviour? It’s like he’s so angry and just can’t help himself

Have an unkind words chart. Every time he says or does something unkind he gets a sad face sticker and after X amount of stickers that means he doesn’t get IPad time, pudding, whatever would be a big loss for him.

Also have a kindness chart and when he does or says something nice he gets a happy face sticker and gets a reward after X amount of stickers.

I'm not sure the right way or best way but I will send mine to bed if I hear too much unkindness..then it's back down for apologies ..it's a routine that works for us as it nips it in the bud now