hey mommyoftwo,
i've been mulling this over and trying to decide whether to respond or not since i wasn't sure i could offer much beside commiseration. something about kewb's response clicked with me, though.
when dd1 has a friend over she will often go down in the basement playroom and usually dd2 is content to stay with me, but sometimes she wants to go down, too, and i do tell dd1 that she has to include her then. if dd1 and friend were in her room that's a little more of a grey area for me since that is not "public" space. if dd1 is being mean about it, though, i am more likely to insist on them including little sister 'cause i'm not fond of exclusion for the sake of exclusion. at dd1's school they have the terminology of "elastic" games. all games are elastic and can be stretched to include everybody.
just not sure how i would approach it with going in her own room, though, since that does seem like her space. dd1 doesn't spend much time in her room and ostensibly she and dd2 share it, but of course they both sleep in our bed. i think i might try (in my rare patient mom moments) to think about dd's motivation. do you think she's purposefully being mean and wants to exclude him (always a possibility in our house) or do you think she just wants some space to play by herself with her friend or some combination of the two? you could address it like you would any issue of ...ermmm... "politeness" (can't think of a better term). for example, when i offer dd1 something to eat at the dinner table and she says, "yuck, gross, bleeeeahhh!" and maybe hurting my feelings instead of getting the message across with a "no thank you" and sparing my feelings. is there another way dd could have her space with her friend w/o hurting ds's feelings? other language or actions? maybe she just needs a little scripting -- though if she's like my dd1 she wouldn't follow the script, but at least you'd have something to fall back on. hope that makes some sense.
i think kewb's idea of each having a friend over makes sense. any little boys or little sisters in your neighborhood?
got to go help dd1 make a puppet theatre while dd2 naps...