DD keeps on hitting her brother

[Errmmmmmmmmmmm]

just that really. DD is 4 and DS is 8. When she gets annoyed with him she hits him. we've told her it isn't nice to hit when it happens, or told her to not hit, given her reflection time but it still continues.
only happens at home, she doesn't hit anyone at nursery.

Any advice?

Make sure he has a space she's not allowed in (e.g. bedroom) without DS's permission.
Remove her immediately and make her stay with you wherever you go and whatever you do. If she complains you tell her when she's a big girl and has learnt not to hit then she can stay playing whilst you do[boring activity] but for now she has to stay where you can see her.

Thank you for your reply. I will give that a go.