Aggressive behaviour solutions

Around 18 months DD1 started hitting, we thought all fine and good we'll deal with it, it's just a phase. But six months later it's worse, not better and both DH and I are at our wits end. We don't smack (personal choice) and there's no family violence here or anything so she can't be modeling our behaviour; she does go to creche but they're very good at keeping on top of kids hitting etc. and I've discussed it with them many times as we work together to try and come up with solutions/be consistent.
What confuses and upsets me is that there doesn't seem to be a trigger for her. She routinely slaps her baby sister in the face, really hard. As in, she'll be walking through the kitchen and DD2 will be in her bouncer, not crying or anything, and DD1 will reach over and smack her face. This happens at least once a day, and that's with us being super-vigilant and pre-empting many other attempts.
It's not just DD2 either - we can be out in public, like at the airport and if DD1 sees another kid she'll often walk over to them and grab them and push/pull them until they fall over for no reason! She kicks DH in the face when he leans over her bed to kiss her goodnight and runs up and bites my legs.
I will put a disclaimer here that probably 50% of the time she's absolutely gorgeous and a wonderful kid. She has a huge personality and at nearly two is speaking in whole sentences and very clearly, people often think she's much older than she is.

Is this aggressive behaviour just a (very long) phase? Has anyone else come up against it with their kids? I just don't understand what is causing it and how we can get her to stop. We've tried redirection, ignoring it, telling her off, taking favourite toys off her, putting her in time out (which doesn't work) and paying lots of attention to DD2 when she's hurt....I'm running out of ideas and half the time just want to cry that my beautiful girl is doing all of these horrible things. Please help! Should I be looking at taking her to see someone? Even as I type that I think it sounds ridiculous - she's only two!

Fi, I've actually been thinking about something like that. Was it a specialist child chiro or just a general one? Did you get a recommendation or call around? I was thinking cranial osteopath, but I think it was the chiro you took Elle to that got me thinking. I'm willing to try anything at this point! We do the 'gentle hands' thing with her to show her how to treat people every time she hits/kicks etc. and she gets it (even nicely stroking my face when she says sorry each time) but then thirty seconds later she'll go and hit again
We have trouble with thinking time/time out because she doesn't care - if she's in a spot in the hallway where there's nothing to do she thinks it's a game (I've spent up to an hour putting her back there again and again and again), if we put her in her room she plays in her bed (there's no toys in there anymore). Just doesn't work for her.

Thanks for replying too - I was beginning to think I was on my own with all the views of this thread but no replies!

Edited by Mazzy

My Sienna is really aggressive too, altho thankfully seems to be only with her sisters, mostly Mercedes but occasionally Maya and Chiara too and she mostly does it out of frustration.
I have no idea what to do about it, we've tried time out and that doesn't seem to work. It fixes the immediate problem by taking her out of the situation but it doesn't seem to be teaching her that it's an unacceptable behaviour.
We're seeing a paediatric psychologist with them (long story) and I just had to fill in a questionnaire for them about the girls behaviours and I mentioned Sienna's aggression, I'll let you know what they suggest as a way of dealing with it next time we see them.

Jake used to be very similar. the "best" one was when my mother was admitted to hospital for emergency surgery. Jake was with us and ran up to a random doctor as he walked past, and kicked him in the shins. Charming.

at the wrehouse we had to be careful about keeping him away from other kids as he would randomly smack them as we went past.

We didn't really do anything that you haven't mentioned, and gradually he grew out of it, and now he is a very social kid - he makes friends very easily, so i guess for him it was a (very long at the time) stage. Oh, the first time we met up with Nikkiwhyte's little fgirl Hannah, I think he whacked her...and perhaps bit. Now they are fine.