Hi Zee,
Wow...step-kids. Can I ever relate.

Having been a step-child and a current step-parent I can relate to both sides of this coin. You came into the picture when your step-daughter was 15...and now she's 17, a horrible time for teenage girls...they're struggling with who they are, peer pressure, school, image, and add a step-mom into the mix and they're really going to have issues. Add step-sibling teenagers and whoa...dynamite. Any chance the family could get step-parenting counseling? Or at least you and your husband? That would be my first suggestions.
My 2nd would be support your husband and your step-kids's graduations with your head held high. Be strong and show that ex-wife of his that you haven't been beaten down and in hiding. Yes, it's tough, yes it's not what you want to do, but if you love your husband you'll do it for him. He's got to do it for you, too, one day and you want him to be by your side.
My 3rd would be, don't try to love your step-children, just try to be their friend and let the relationship develop how it will. It takes step-families 3-5 years before relationships begin to mesh well together...sometimes longer, sometimes it just doesn't work out, but it's normal, I think, to have stronger feelings for your own children. I'm a step-mom to 2 boys and a girl...all in their late 20's when I met their father. It's not been an easy experience and there's been some downright disrespect shown to me by two of the three so much so that they walked away from their dad and me for awhile. The important thing to remember is you and your husband must always support and stick by one other no matter what. Be on the same page about decisions concerning the children (at least in front of them). If your children or his children find they can put a wedge between you and your husband it will only start to chip away at your relationship and that's not going to help your marriage at all. My step-kids tried that early on and they had to learn the hard way that this was not something that was acceptable nor tolerable and Larry stood by me...they'd been so used to manipulating their dad, so when I married Larry and would not let them do that to him any longer it caused a lot of problems for awhile...but, eventually, they came around and now five years later we're doing pretty well considering. It just takes times, patience, standing firmly beside your husband and him standing by you, and the kids learning they cannot push your buttons and it have an affect on you.
It's still too early for all of the relationships to be at a place where everyone is going to blend well. And the teenagers alone are enough to make issues on their own, not just in a step-situation.
My heart goes out to you and I'll be curious to see how all this works out. Keep us posted and stay strong for your husband and your step-children and your children. Best of luck, sweetheart...best of luck.