The military
His middle name is Douglas.
An internet search did not find any US President with Milton as a middle name. President Nixon's middle name was Milhous.
Theodore Roosevelt, 26th US President, had no middle name.
Anne Hutchinson, nee Marbury, did not have a middle name. This was not uncommon for the time period, as middle names did not become commonplace until after the American Revolution.
Washington
As a middle name, I'd say Suzanne. Jenna Suzanne. As a sister, I'd say Rachel. Jenna and Rachel.
George Walker Bush
Jenna Dolan's birth name is Jenna Hahn.
Jenna McDanold's birth name is Jenna Sjunneson McDanold.
Jenna Ushkowitz's birth name is Jenna Noelle Ushkowitz.
Jenna Coleman's birth name is Jenna-Louise Coleman.
Jenna Bos's birth name is Jenna E Bos.
Jenna Stern's birth name is Jenna Louise Stern.
Jenna Morrison's birth name is Jenna Rose Morrison.
Jenna Golech's birth name is Jenna R. Golech.
Barbara Pierce Bush