Proto-Indo-European Roots

Root/Stem: *awo-, *awio-
Meanings:  mother's brother
Cognates (32):
Hellenic Greek aia (ground, motherland)
Italic Latin avus (grandfather)
Celtic Common Celtic *avios (related to grandfather); > 
Old Irish aue, haue (grandson), Irish ó (grandson), Welsh ewythr (mother's brother), Cornish euitor, Breton eontr
Dardic & Nuristani
Armenian Armenian hav, gen. havu (grandfather)
Germanic Common Germanic *awón-, *awé-, *awun-; > 
Gothic awó (grandmother), Old Icelandic ái (great grandfather), afi (grandfather), Old English & Old Saxon éam (grandfather), Old Frisian ém, Old High German óheim (mother's brother), Middle Low German óm
German Oheim (mother's brother), Dutch oom (grandfather)
Baltic Lithuanian ava (mother's brother's wife), avynas (mother's brother), Old Prussian awis 
Slavic Common Slavic *ujï (mother's brother), > 
Ukrainian vuj, Old Serbian  uika, Bulgarian uiko, Serbo-Croatian ujac, Slovene & Czech & Slovak ujec, Polish wuj (uncle), Lower Sorbian hujk (uncle, cousin), Russian uj (mother's brother)
Notes: The term is rather dark, and its origin and exact meaning are not quite clear yet. It is obvious, however, that it denoted kinship. Most of modern Indo-European languages do not preserve the original complicated system of kinship terms: uncle in English means both 'mother's brother' and 'father's brother', while in Proto-Indo-European there used to be two different words. The same goes for other terms: 'sister's husband', 'son's wife', etc. 
The presence of the initial *H- (laryngeal) is also a question.