"a month ago she repeatedly punched me in the face because of a tv remote."

The girl is violent? AND your husband doesn't believe you?

Run, do not walk, back to where you came from. No one has to endure and abusive relationship. Ever. Get out and don't look back.

I'm sorry if you misunderstood me.

My husband did believe me - he is amazing and stands by me with everything. The eldest daughter understands that if she even touches me in a violent way i will call on the police this time, she is also on probation and we did inform her PO about what happened and we are currently doing counselling sessions because of this. Her behaviour has gotten better and is more tolerable than the youngest.
My main issue as I'm sure you can read is with the youngest daughter.

It sounds like you are handling things well with the oldest -- Good for you! And it's also very positive that your husband is standing strong and supportive beside you. As a united team, you can remain strong together and will have a chance to turn around the younger daughter.

I wish I had some magic words that would fix the younger daughter's attitude -- but of course, I don't. The only tactic I could suggest is one I use to prevent my two boys from fighting, and that's just to nip it in the bud. At the first sign that things are starting to turn sour, I just separate the two, each to a different room, until they can agree to behave civilly. Of course, mine are a bit younger. Does the counselor you're seeing have any suggestions?

So I'm just going to vent today... I need to - I'm getting really tired of the attitude that Keri gives constantly - and if its not towards me its either towards her dad or her older sister, its like she enjoys it and she doesn't really care who she upsets in the process, she hardly shows any respect for her dad ( my husband) that really gets to me - its like he did nothing to deserve that, he wasn't the one that deserted her at a young age, he was the one that took her and her sister in, gave them food, a roof over their head, clothes and anything they wanted when he could afford it. Heck the mother of the girls has over $10 000 owing in child support - but yet when Keri gets paid who does she think of to take out to eat first.. her mother and the boyfriend & her half sister - nice hmmm.
I am really at my wits end with that child - I don't know what I should do. I've tried to ignore her ( but when she has done something wrong she comes back all sweet & innocent and sucks up - and that is one thing that I can't stand it - no apologies to anyone nothing) If i try to reason with her her attitude just gets worse and she acts out - worse than a 10 year old child with bad temper tantrums, it really is an ugly sight to see especially when she is 15 years old - i mean 5 more years and she's 20 does this mean that when she's 20 she's going to behave like a 15 year old? O Lord help her then!
I don't know, like I said I'm at my wits end do I really deserve this? I love my husband dearly, he is truly the love my of my life and someone that I can't live without.

The only thing that I can think of, that has helped me immensely is that anger is the easiest emotion to show, at least for kids it seems. Taking her mom out to dinner? Of course!!! One thing I've noticed in my situation is that the kids are ALWAYS seeking their mother's approval, no matter what.... I would imagine even more so if they have been deserted by her.
So back to the anger, if you can figure out what's really going on (aside from the turmoil change causes), and more importantly, don't take any of it personally, it's going to work wonders.
The bummer about step-parenting is that you have to earn love, respect and all those things that biological parents get automatically. And sometimes it seems as if, not matter what you do, none of it goes recognized. I am amazed by how much those boundaries are tested on a daily basis!
Step-parenting, much like parenting, is a trial and error process, ours with much much more scrutiny. So if it doesn't work, put it in a folder of not successes and file it away, that same strategy might work later on.
My coping strategies? Close myself in the bathroom and cry is the first step, take a shower (I've been known to take multiple showers at times) if water soothes you, pray (or meditate, whatever spiritual practice you may have), find support outside of your family (my husband is my greatest ally, but my mom group is also a great help), give yourself a time out or time to do something you love.
I don't know if any of this helps. I've been at it for two years now, and with MUCH work on everybody's part (the children included), things are getting better. It's not time that fixes things, it's what people do during that time. Your girls have a choice to make, as teenagers it's easy to choose pain over pleasure, especially if they've known pain at an early age, but as I say to our kids, "There is always an alternative".
So hang tight, remember that it's just a phase, and know that you can count on all of us out here!