bʰréh₂tēr-yo-
“of a brother, brotherly”fraternal, fraternity, friar
Extended form of *bʰréh₂tēr giving Latin frāternus, English fraternal, fraternity, friar.
Discussion
The PIE form *bʰréh₂tēr-yo- (of or pertaining to a brother, brotherly) is a relational adjective from *bʰréh₂tēr (brother), formed with the suffix *-yo- that creates adjectives of belonging. The meaning is "that which belongs to the brother-relationship" — the quality of brotherhood.
Latin frāternus (fraternal, of a brother) continues the form through the expected Italic phonology: PIE *bʰ > Latin f, with the long vowel from the laryngeal preserved. The English derivatives are institutional: fraternal (pertaining to brotherhood), fraternity (a brotherhood — originally a religious brotherhood, then an academic one), fraternise (to behave as brothers — now often with the negative connotation of inappropriate familiarity with enemies or subordinates), and fratricide (brother-killing — frāter + caedere).
The English word friar (from Old French frere, from Latin frāter) shows the kinship word becoming a religious title: the Franciscan and Dominican friars were literally "brothers" — members of a spiritual brotherhood. The word frère in modern French still means "brother" in both the familial and religious senses.
The suffix *-yo- is the same relational suffix found throughout PIE adjective formation (cf. *dʰwer-yo-, "of the door"; *h₂ner-yo-, "of a man, manly"). It consistently created adjectives meaning "pertaining to X" and survived productively into Latin (-ius/-ia/-ium) and through Latin into English (-al, -ar, -ary).
The fraternal/sororal pair (from *bʰréh₂tēr/brother and *swésor/sister) shows that PIE had dedicated relational adjectives for both sibling relationships — further evidence for the centrality of kinship terminology in the proto-language.