Sadly, sounds like a 2 year old. 
 They just don't have the ability to understand WHY they shouldn't pull hair, hit, etc so they feel the impulse and do it. I would try to get big sister to be a part of avoiding putting herself in a place where/when the little one tends to go for her. Instruct her to hold the little one's wrist firmly but not meanly to prevent a hard yank, give her words to use when she gets hit like "You can't touch my body like that! I don't like it!" Just prevent, prevent, prevent until the little one is older and by then I imagine the hitting and hair pulling will end. Little one ins't meaning to hurt, therefore hurting him/her on purpose to make a point (though I'm not sure what point is ever made when one is hitting a kid....besides "I can hurt and scare you!") is simply mean.
Hang in there Mama!
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