Sino-Tibetan holds the Chinese languages, Tibetan, Burmese and several hundred others spoken across the Himalayan foothills, the uplands of Southeast Asia and southwestern China. By speaker count it is second only to Indo-European, and almost all of that count sits in the Chinese branch. The rest of the family is extraordinarily diverse and comparatively little described: many of its languages have a few thousand speakers, no written tradition and no full grammar, and new ones are still being documented.
Relating them to each other is genuinely difficult. Chinese and Tibetan have both been written for well over a thousand years, but their early written forms are separated by so much time and so much borrowing that reconstruction has to work around loanwords in both directions, and the internal structure of the family is still an open field rather than a settled tree. Even the name has been argued over, since some scholars prefer Trans-Himalayan to avoid the implication that Chinese and Tibetan form a primary division. What is not in doubt is that the family exists, that Chinese and Tibeto-Burman are related, and that the connection was established through exactly the same comparative work that established Indo-European.