As India or Europe or even OTL China's Han migration experience says, ATL Caucasians will overrun Southern China as well as China's geography is flat and the only barrier is the Yellow River. I could see that ATL Caucasian-dominated China will dominate the Southern part.
Cultural domination is different from lingustic unity though. IOTL, while Northern and Southern China speak (more or less) the same language, Northern Chinese are more closely related to Europeans, and Southern Chinese to Australian Aborigines, than either group is to each other.
Thus, while I'd say it's likely that an Indo-European language could dominate southern China, Indo-European genes would not.
Also, look at India. The Dravidian south was almost certainly conquered by the Aryans at some point (given common religion, and the Sinhalese in Sri Lanka). Yet the presumably ancient Dravidian languages survived. True, India never unified in prehistory the way China did, but there was no significant geographic barrier stopping this.