Eudo of Penthièvre was the brother of Alan III of Brittany

Alan III, Duke of Brittany

Alan III of Rennes was duke of Brittany, from 1008 to his death. He was son of Duke Geoffrey I and Hawise of Normandy. Alan married Bertha, daughter of Odo II, Count of Blois and had at least two children: Conan II, his successor, and Hawise of Brittany, who married Hoel of Cornwall.Alan III was...

 (1008-1040). Eudo had been loyal to his brother throughout his reign, but he subsequently spent many years trying to wrest ducal power in Brittany

Brittany

Brittany is a cultural and administrative region in the north-west of France. Previously a kingdom and then a duchy, Brittany was united to the Kingdom of France in 1532 as a province. Brittany has also been referred to as Less, Lesser or Little Britain...

 from his nephew and ward, Conan II

Conan II, Duke of Brittany

Conan II of Rennes was Duke of Brittany, from 1040 to his death. Conan was the eldest child and heir of Alan III, Duke of Brittany by his wife Berthe de Blois, and member of the House of Rennes...

 (1040-66). Eudo was not finally defeated until 1057, but the loyalty of both his eldest son Geoffrey I Boterel

Geoffrey I Boterel

Geoffrey I Boterel was a Breton noble and one of the leaders of the Breton Revolt. He was the son of Eudo of Penthièvre, younger brother of Alan III of Brittany. Eudo had been loyal to his brother throughout his reign, but he subsequently spent many years trying to wrest ducal power in Brittany...

 and his nephew Geoffrey Grenonat

Geoffrey Grenonat

Geoffrey II Grenonat, Count of Rennes, married Bertha, daughter of Rivallon I of Dol and Aremburga de Puiset. Alongside Ralph of Gael, he revolted against Duke Hoel. Geoffrey played a commanding role in the Breton Revolt, in which he and several others planned to restore the authority of his uncle...

, count of Rennes

Count of Rennes

The Count of Rennes was originally the ruler of the Romano-Frankish civitas of Rennes. From the middle of the ninth century these counts were Bretons with close ties to the Duchy of Brittany, which they often vied to rule. From 990 the Counts of Rennes were usually Dukes of Brittany...

 (1066-84), a natural son of Alan III, remained dubious until their respective deaths, in revolt, in 1093 and 1084.