That would be more like Mandarin to Cantonese, both are Han Chinese.
Lao, Siamese, black Tai, White Tai, Shan, Tai lue, White Tai are Tai-Kadai. Thailand and Laos are Tai Kadai speaking countries.
http://en.wikipedia.org/wiki/Tai_peoples"Thai peoples" redirects here. For the subgroup of the Tai, see Thai people
The Tai or Tai-Kadai ethnicity refers collectively to the ethnic groups of southern China and Southeast Asia, stretching from Hainan to eastern India and from southern Sichuan to Laos, Thailand, and parts of Vietnam, which speak languages in the Tai-Kadai family and share similar traditions and festivals, including Songkran.[1] Despite never having a unified nation-state of their own, the peoples also have historically shared a vague idea of a "Siam" nation, corrupted to Shan or Assam in some places, and most self-identify as "Tai".