It sounds like your 22 month old is going through a normal stage, and the hitting can be caused by lots of different things including (but not limited to):
* frustration at not being able to verbally express her needs/desires --> maybe try sign language with her? Signing Time DVD series is an excellent sign language course for babies/kids that really help reduce frustration by giving them tools to communicate. You can usually get for free from the library, or watch for free on your local public television station (http://www.signingtime.com/pressroom/stationcarriage.html), or order. We actually ended up liking them so much we ordered them - great for teaching parents sign language, too!
* frustration due to physical discomfort -
A) hunger - last meal or snack? try and offer a healthy, whole foods snack like a few slices of apple, 1/2 a banana, carrot sticks, etc.
B) exhaustion - not getting enough rest? had a particularly active day? she might be overtired and lacking her normal levels of self-control - offer some down time in your lap with a book or toy
c) having to pee/poop - if your 22 month old isn't yet potty trained, the next time she starts to get short tempered/start hitting, take her to the bathroom and offer to read her a book while she sits on the potty. Make no big deal out of it, tell her to try to make pee pee come out, start a little running water in the sink, and then begin reading the book or encourage her to play with a toy. Give her a good 5 minutes of time on the potty (while distracting her with a book or toy, etc), and see what happens. Maybe nothing the first 5 times, then something. Maybe she'll go for you the first time. Children have windows of toilet training readiness - she's at one of the typical ages for our culture (in other cultures, they start from birth, or start between 3-6 months, etc. - so you're not going to be rushing her by sitting her on the potty as long as you do it in a NO pressure, non-coercive, relaxed way). Anyway, many kids get particularly cranky when they feel the need to pee/poop - they feel the pressure/discomfort, but don't quite know what to do next (which is where you come in). ;)
Those are just a few things to keep in mind and try. This, too, shall pass - hang in there, respond with limits, compassion, and teaching --> show compassion for the older sister who was hit ("Honey, I'm sorry sissy hit you! Ouch! That must have hurt!"), state the limit/need ("You look frustrated/angry! We don't hit/pull hair. Hitting hurts, ouch! Gentle hands, please.") And physically model the right behavior for her.
Speaking of frustration, my 12 month old is "calling" for me!