Newt Gingrich's lesbian half-sister Candace Gingrich-Jones says she is disappointed that he signed a pledge to work against gay marriage.

Last week, Gingrich vowed to defend the Defense of Marriage Act (DOMA) and remain faithful to his third wife, Callista, in agreeing to the principles behind The Family Leader's controversial 14-point anti-gay marriage pledge.

Gingrich did not sign the pledge, but in a written statement to the Iowa-based group headed by Bob Vander Plaats, Gingrich promised he would “defend and strengthen the family.”

“As President, I will vigorously enforce the Defense of Marriage Act, which was enacted under my leadership as Speaker of the House, and ensure compliance with its provisions, especially in the military. I will also aggressively defend the constitutionality of DOMA in federal and state courts. I will support sending a federal constitutional amendment defining marriage as the union of one man and one woman to the states for ratification. I will also oppose any judicial, bureaucratic or legislative effort to define marriage in any manner other than as between one man and one woman,” Gingrich wrote.

Later in the week, he signed the National Organization for Marriage's (NOM) 5-point pledge to work against marriage equality.

“I am disappointed that he has succumbed to the Family Leader and this pledge, even though he didn't sign it,” Gingrich-Jones told gay glossy The Advocate during a wide-ranging interview.

(Related: Newt Gingrich's half-sister, Candace Gingrich-Jones, says she'll vote for Obama.)