Sibling rivalry

My family has a situation that we need help with. We have three girls 15,
12, and 11. DD2 and DD3 have never gotten along and pretty much DD2 cannot
stand DD3. DD3 is a typical 11 year-old in that she is the annoying little
sister. DD3 does have issues though, she is loud, not too clued in socially,
and has anxiety and lots of fears. We have been to therapists and I have
talked about her before on this list. She is very argumentative and doesn't
do chores when asked. DD1 and DD2 have become much closer over the years
because they have more in common. It seems it is now the oldest two against
the rest of the household. However, the problem we are having is that how
can we stop the hatred dd2 has for dd3. DD1 is kind of neutral, but she will
gang up on DD3 and will be mean also. For right now, we have 4 bedrooms, 2
up and 2 down. Hubby and I share one small room down, and DD3 has the other
small bedroom downstairs. The two older girls have the two larger rooms
upstairs. It has been a bone of contention with DD3 because she feels even
more left out and the two older girls pretty much exclude her in all
activities and they hole up upstairs and will not let DD3 come up to be with
them. Bedtime is especially hard because DD1 and DD2 will stay up (even
though we tell them to go to bed) and will talk and laugh. DD3 thinks this
is horribly unfair and wants to be moved up there with them. No one wants to
share a room with DD3. We thought of moving DD1 down to our room and hubby
and I move upstairs, but I really don't want to do this because of I am
uncomfortable with having the kids downstairs and me upstairs.
I just don't know what to do anymore. They fight constantly and it truly is
putting a strain on hubby and I. How much should hubby and I be in their
fights? We intervene if they hurt each other, but when DD3 cries because her
feelings are hurt once again by being left out, what should I do? I just
cannot stand all the hatred this family seems to have. Thanks for any input
anyone can give me.

Sue (mom to three girls)