Sometimes, I read passages like:
Languages X, Y and Z in region A are closely related to each other, comparable to French, Italian and Spanish in Western Europe.
The discussion in the question "Do distantly related languages have a lower incidence of false friends?" also implies that languages can be considered "closely related" or "distantly related" to each other.
How would one measure such distance and do linguists make that measurement for any purpose? Sometimes it's logically obvious, as a commenter in the linked question point out, that "it is uncontroversial to say that, given German as a reference point, Flemish (both West Germanic languages) is closer than Danish (both Germanic languages), which in turn is closer than Italian (both Indo-European)". How about comparison across different families? For example, How would one determine if Mandarin and Cantonese are (relatively) more closely related to each other than French and Spanish?