The claim is true as a broad characterization of the English Civil Wars: historians describe the conflict as one in which “brother took up arms against brother,” and where “fathers and sons, uncles and cousins were pitted against each other.” That matches the idea that the war pitched brother against brother, even though it is a summary rather than a statement about every individual case. (books.google.com) Sources: Nick Lipscombe, *The English Civil War: An Atlas and Concise History of the Wars of the Three Kingdoms 1639–51* (Google Books); Tristram Hunt, *The English Civil War At First Hand* (Google Books).