Old English broðorlichnes, in a literal sense, "state or quality of being brothers;" see brotherly + -ness. The meaning "quality of being fraternally kind or affectionate" is from 1530s.
Entries linking to brotherliness
brotherly adj.
Middle English brotherli, "between brothers," from Old English broðorlic "of or pertaining to a brother;" see brother + -ly (1). The meaning "fraternal, kind, affectionate" is from 1530s.
-ness
word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).