We have been struggling with LO1 (two) hitting her little sister when she becomes frustrated with something LO does or something we aren't letting her do and she takes it out on her sister. DH and I usually firmly tell her that is not ok and put her directly in a time out. We make her apologize and give her sister a hug afterwards. I'm not really sure this is working because she still does it a few times a week. A lot more often when she is tired or not getting what she wants, which is toddler behavior but not ok.
For those of you who have dealt with hitting, what is your best advice for handling it? Is it something we will just have to deal with because of her age or is there a way we can fix this issue?