purple's experiences sound very similar to mine. I have 2 DSSs, 11 and 9, and 2 DSs, 8 and 6. We live all together 50% of the time. DSS1 and DS1 have taken several years to get to a stage where they sometimes play together, but more often than not do not choose to. Lots of poking, prodding, winding up by DS1 to get DSS1's attention. Lots of sarcastic comments and snideness from DSS1 to DS1, and some lashing out and hitting when DS1 gets really irritating. Lots of tears here too. There has been, and still is, resentment on both sides, although it is lessening now. And of course, DP and I feel guilty sometimes that we put them all in this position.
There are all sorts of things tangled up in this I think - DSS1 and DS1 were both used to being the eldest, and it has taken DS1 a long time to adjust to being 3rd in age out of 4 when he's here. The "pecking order" between the 4 of them needed to be established. DSS2 and DS2 had always been the youngest, and this has seemed to be easier for them than for DSS1 and DS1. Also DSS1 sometimes gets jealous if DS1 plays with DSS2 - they get on pretty well most of the time. That has eased as DSS1 has got older, and develops more of his own individual interests. We do the same in the car, we don't encourage them to sit together, although we have tried getting them to do activities together which has worked on occasion.
The other day, DSS1 shouted specifically for DS1 to come and see something funny on the computer - again like purple, that wouldn't have happened 6 months ago. Also DSS1 has, from his own choice, gone out and bought small gifts for all the others for christmas this year, including DS1. You have to grasp the small signs that they are seeing themselves as a family!
I read in a step parenting book that it can take years for relationships in a step family to shake down. I think this is so true. DP and I sometimes joke that they'll all settle down and sort out their pecking order just in time for them to start leaving home!