Proto-Indo-European Roots

Root/Stem: *bhrátér-
Meaning:  a brother
Hellenic Greek phratér (a brother), phrátór (a member of the brotherhood), phratria (a community)
Italic Latin frater (a brother), Oscan fratrúm (a brother;, Umbrian fratrom (a brother;, frater (brothers;; 
French frère (a brother), Romanian frate, Megleno-Romanian frati, Istroromanian frote
Celtic Common Celtic *bratér, >
Gaulish Bratronius (personal name), Old Irish brathir, Irish bráthair, Scottish Gaelic bràthir, Welsh brawd, pl. brodyr, Cornish broder, pl. bredereth, Breton breur, breuzr, pl. breudeur
Indic Sanskrit bhrátár- (a brother), bhrátra- (brotherhood)
Dardic Khowar brar (a brother)
Iranian Avestan & Old Persian brátar- (a brother)
Anatolian Lydian brafr- (a member of the community)
Armenian el'bair (a brother), gen. el'baur
Tocharic Tocharian A pracar, B procer (a brother)
Balkan Illyrian bra (a member of the community); 
Albanian vla (a brother) ?;
Venetic vhraterei dat. 'to a brother'
Germanic Germanic *bróder (a brother), > 
Gothic bróþar (a brother), Old English bróþor, Old High German bruoder, Old Norse broþir
German Bruder, Dutch broer, Scots brither
Baltic Common Baltic *brat- (a brother), > 
Lithuanian brolis, Zhemaitian bro'tis, Latvian bralis, Old Prussian brati, brote (brother, brothers), Sudovian bra'te'
Slavic Common Slavic *bratü (a brother), > 
Russian brat, Old Church Slavonic bratru, Czech & Upper Sorbian bratr, Lower Sorbian brats', Polish & Slovene & Belorussian & Ukrainian brat
Notes: With this root we begin a series of Word-A-Weeks related to family members. This set of words denoting relativeness is very popular in introductory Indo-European courses - I believe words like this are the best example of the common roots of all IE languages. They all - 'mother', 'father', 'sister', 'brather', 'daughter' - were declined in the same way with the suffix *-er which is preserved in English as well. Another characteristic feature is that these roots are very stable in all branches, so even now they sound the same in a lot of different languages.
In many languages of the East (Altaic, African etc.) there are different words for terms like 'elder brother' and 'younger brother'. Indo-European has only one brother, and this kind of analytic trend existed already in the Proto-language.