Two new books extend the oft-told stories of Franklin Delano Roosevelt and his fifth cousin, Theodore Roosevelt, via a look at sons, daughters and webs of familial relationships.

In William J. Mann’s “The Wars of the Roosevelts: The Ruthless Rise of America’s Greatest Political Family” (Harper, 609 pp., $35), it’s Hyde Park vs. Oyster Bay, upstate vs. Long Island; FDR and the effete Roosevelts of the Hudson Valley vs. TR and his hardy brood out of Sagamore Hill, in a saga that divided a family with century-old roots in the Empire State.

In “His Father’s Son: The Life of General Ted Roosevelt Jr.” (New American Library, 342 pp., $27), Tim Brady details the life of Theodore’s scion as he struggled to live up to the legacy of his formidable father.

If Brady is sincere and dutiful to a fault, Mann sniffs around his material like a seasoned tabloid reporter on the hunt for a good story. There’s lot to work with, and Mann has mined troves of material — letters, court records, diaries and other sources. Scandal framed this tale from the start, as TR tried to contain his wayward brother, Elliott, from bringing ruin to the family name in the early 1890s. An affair with a servant, Katie Mann, had left her pregnant. The family settled to keep the woman (no relation to the author) quiet. Here began the ne’er- do-well Elliott’s decline into drink and death in 1894.

For the author, unruly figures such as Elliott were broken on the wheel of rigid ambition — the singular trait, he argues, of both Roosevelt branches. Another casualty was Eleanor Roosevelt, Elliott’s beloved daughter. Homely and plain compared with her more glamorous cousin, Alice (TR’s eldest child), Eleanor struggled to find a place at home and in the world. She never quite fit in with the TR branch: She adored her father, and his downfall was traumatic. Her marriage to Franklin Roosevelt in 1905 furthered conflict between the family wings.

TR may have served nearly two terms as president, but it was FDR who claimed the family’s progressive credentials for the Democrats. (Or usurped them, as the ever-seething Alice saw it.) Eleanor was instrumental in FDR’s rise, yet she never quite fit in as wife, either. Mann sensitively chronicles her intense relationships with other women, which provided refuge from Franklin’s ambitions and infidelities.

advertisement | advertise on newsday

Mann is with the outriders and black sheep. Indeed, he brings one such figure out of the shadows, Elliott Roosevelt Mann, Elliott Roosevelt’s illegitimate son. With no Roosevelt money or connections, the younger Elliott forged a life as a banker and lived in Queens. “Had he been born legitimately, or had his father’s family embraced him, Elliott would have done the Roosevelt name proud,” Mann observes. He contacted his half-sister Eleanor by mail in 1932; she did not respond. “Eleanor simply could not share her father with anyone else,” Mann writes, “least of all those he abandoned even more grievously than he abandoned her.”

FDR’s rise would not go unchecked by the Oyster Bay Roosevelts. Alice became a fount of opposition to FDR’s New Deal as an influential columnist and political operator. But it was the duty of Ted Roosevelt Jr. to succeed as TR’s political legatee. In this, he was a failure. Still, in “His Father’s Son” Brady writes sympathetically about Ted and his political endeavors.

It was said of TR that he was a dilettante as a soldier and a pro as a pol; the reverse was true of Ted Jr. He served in the New York State Assembly in the early 1920s, and later as governor of Puerto Rico, and campaigned against FDR when he stood as vice president on the losing 1920 Democratic ticket. A loyal Republican, Ted nonetheless had no place in a party that had abandoned his father’s liberal nationalism for the business-first values of Harding and Coolidge.

On the battlefield, however, Ted Roosevelt Jr. shone. Brady’s best chapters follow Ted first in World War I, where he was wounded and decorated, then in World War II, when he became, at 53, a general. Beloved by his men and known for his disheveled uniform, he was the oldest soldier ashore on D-Day. His troops landed way off target, but he rallied them with the famed quip, “Gentlemen, we’ll start the war from right here.”

Sadly, he died of a heart attack in July 1944. Still, there was plenty of TR’s grit in his warrior son.