Which is the most correct?
Jean is the friendliest of both brothers
or
Jean is the friendlier of both sisters.
Neither of the two sentences:
- (1) Jean is the friendliest of both brothers.
- (2) Jean is the friendlier of both sisters.
is really good, natural English.
The first issue is that the superlative "friendliest" is not used unless there are three or more choices. (I recall a line in a novel that a woman saying "My dearest husband" implied (incorrectly) that she had at least three husbands.) this issue occurs only in sentence (1).
The second issue is that "both" is not used in that sort of construction. a more natural version would be:
(2A) Jean is the friendlier of the two sisters.
this would work just as well with "brothers" or any other noun indicating a group of people, for example "friends".
By the way, Jean is female in English. Jean is only male in French.
The problem is the word "both". It doesn't belong here.
Jean is the friendliest of the brothers
Jean is the friendlier of the sisters.
I think "the friendlier" is mostly used where there are two, and "the friendliest" where there are more than two.