Are Cantonese and Lao related?
Lao is a member of the Tai–Kadai language family. This family does include some languages spoken by minority groups, including the Zhuang languages of Guangxi. Several languages in this group have words which sound familiar to Cantonese speakers. (I remember traveling to Thailand and being shocked by how similar Thai numerals are to those of Cantonese.) Nevertheless, the historical relationship of this language family to Sino-Tibetan (the language family which includes Chinese dialects like Cantonese) is a matter of some debate. From Wikipedia:


  The Tai–Kadai languages were formerly considered to be part of the Sino-Tibetan family, but outside China they are now classified as an independent family. They contain large numbers of words that are similar in Sino-Tibetan languages. However, these are seldom found in all branches of the family, and do not include basic vocabulary, indicating that they are old loan words (Ostapirat 2005).


That these languages share many similar words is not surprising because of the historical proximity of these communities, but it appears most linguists believe that there isn't a deep connection between the language families.