The book is about three kid/teen siblings, an older sister and two younger brothers. They have the ability to mentally travel to another world using dreams/telepathy but they have to be holding hands to do so. One of the brothers stutters in the regular world but doesn't stutter in the other dream world. They've been to the dream world a few times but on the next visit the brother who stutters has an argument with his sister and runs off not wanting to return home.
His running off strands them and endangers their lives as their bodies back on earth can not function for too long without them and there is something about the air of the dream world that is dangerous to them if they stay too long and it is the same with the food and water. A creature called Miascus(sp?), shows up and is able to speak with the kids, or maybe just one of them, but Miascus is sent/volunteers to go find and bring the runaway brother back and does so, possibly for some kind of favor in return.
There are other creatures, humanoids I think they were, the females might have laid eggs, and could fly but not very high and they were always trying to fly higher for some survival reason. The siblings, led by the sister, are able to briefly let one of the females of the creatures fly high enough to see above the clouds/dust/fog that is constantly present and making it difficult for all of them, the siblings/Miascus/and other creatures to survive.
The creatures have a strong reaction of some kind and so does the Miascus. One wants to keep the kids there the other wants to allow them to leave. The siblings have to rush to escape so they won't become prisoners there in that world.
They make it back home to their physical bodies that have been been declining from how long the kids were gone, I don't remember how long, they are cold/stiff/numb when they re-enter their bodies/awaken from the sleep/dream world and have to spend some time recovering.
The older sister and brother are still having trouble getting along but try to do a better job. Some other kids who had been bullying the brother about his stuttering show up and to stop them from teasing him starts telling them about the dream world, but without telling them about his and siblings abilities so the bullies think he has simply made up a cool story which is being turned into a cool game for them to play.
I don't remember anything else about the end but it is a book that I saw years ago, probably late '90s to early 2000s. Don't know if that was the time frame of its publication though. I've looked for this book for years online and in used bookstores but haven't been able to find it. Hoping someone here can help me find it as it was a really interesting book.