My adopted children were older than my biokids too, and that caused some extra issues, because the bio kids were able to handle tons of things the adopted kids couldn't. Here's a good post I did for a lady adopting a RAD child older than her bios (different age, but kids of trauma are often younger developmentally).
My son was very violent to everyone and my adopted daughter was aggressive toward my biodaughter (jealousy, pushing to get kicked out like everyone else has done, delayed emotional and socially - so tended to be more like a toddler who uses her hands not her words...). For a long time we tried to treat the children equally, especially the girls.
Here's some of the things we did (or I wish we had done sooner!).
* We sat the adopted children down and tell them that they were being treated differently because of their trauma, not because we loved them less or loved biokids more. Trust Jars/ Love Jars post - http://marythemom-mayhem.blogspot.com/2008/11/love-jar.html
It was our job to keep everyone safe. We cared about them, but the younger biochildren did not have the same childhood and there were going to be areas where they got to do things the adopted kids didn't, even though they were older. None of this solved anything by the way, but it gave us a reference point that we could keep pointing back to, "I know it doesn't feel fair that your sister gets to spend the night at your friend's house and you don't, but you're not at a place to do that right now, because it's hard for you. Your sister didn't have the trauma that you did so she can handle it. You'll get there! Just not today."
*Stop treating them equally. They are not equal! They have different life experiences, different interests, different needs... My mom always emphasized that with my sister and I. We both got a Christmas present, but it wasn't matching dresses (which is one thing my dad liked to do)! Neither of us would want what the other wanted!
^This is actually how the FAIR Club got started.^ http://marythemom-mayhem.blogspot.com/2009/03/how-to-discipline-your-difficult-child.html My kids were constantly whining, "That's not FAAAIIIRRR!!!" I needed a way to discipline and structure their lives that was appropriate for each of them. Structure for the adopted kids, but not really punishing them for things that were out of their control (fight/ flight/ freeze reactions for example) - while avoiding letting the biokids feel that the adopted kids were "getting away with" behaviors that we didn't want the biokids to start thinking was OK for them to do!
*Remember that develomentally (emotionally and socially) kids of trauma are a LOT younger. http://marythemom-mayhem.blogspot.com/2011/04/developmental-stages.html If we expect them to "act their age," we're all going to be disappointed.
*ABSOLUTELY no touching. None. Ever. The violent one especially, literally had to be out of arm reach of the other kids at all times. If I had to be in another room then that child came with me or was in his/her room alone. They weren't allowed to sit next to each other on the couch or in the car. They were NEVER allowed to be alone in the same room.
* Separate rooms. Originally the girls shared a room. BIG mistake! We converted the playroom to a bedroom to separate them. When I was a kid, my bedroom was the breakfast nook with some slatted closet doors bolted in to make a wall.
*ABSOLUTELY no parenting. The adopted kids felt they had a right to boss the biokids around and the biokids were good kids they just took it. EVERY time we heard it, we reminded everyone that WE were the parents and that was not their job. We NEVER put the kids in a position where they got to tell the other kids what to do. Not even relaying a message ("Mom said to come downstairs and do the dishes.") At most, they were allowed to say, "Mom is calling you."
* Individual parent time. Just you (or Hubby) and the child doing something together. Could be making a meal, going shopping, a "date," sitting next to their bed and chatting, telling a story or singing... I tried to make it fun, even when they were being obnoxious, awful or in trouble. It was an attachment activity and necessary. I did this with the biokids too, because they deserved a break too.
* Provide structure and reduce overwhelm. Our adopted kids needed LOTS of structure. Their insides are so chaotic that we had to make the rest of life as calm and simple as possible. That means stripping their rooms of all but a bed and one toy. It means chores that biokids could handle have to be simplified and fewer. Multi-step directions were overwhelming and impossible. They usually triggered melt downs.