DW: Yes, John Adams, II. He was the son of John Quincy Adams and the second child of a president to be married in the White House. The bride was Mary Catherine Hellen, his own first cousin.
The John Quincy Adams’ were empty nesters when they went into the White House, all three of their boys were away at college. So the first Lady brought in her nephews and nieces to fill up the mansion. Mary was one of the nieces and she was a precocious flirt who managed to get all three of the Adams sons to fall in love with her and at one time or another they were all engaged to marry her.
When John Adams, II was expelled from Harvard, he came home to the White house, was the last one to fall for her and they were married in what is now the Blue Room, then the Elliptical Room of the White House. Neither of the other brothers came to the wedding and one of them committed suicide a year later.
The groom, John Adams, II died an alcoholic in his twenties. But Mary Catherine Hellen made up for the pain she brought into the family. She lived long and cared for John Quincy Adams and the First lady well into their senior years.
Doug Wead 
From CNN, the article
Bush: Jenna’s wedding was ‘spectacular’
Doug, congratulations on being a main cited source.:)