Whitney Houston Daughter -- to Marry Adopted Brother

Bobbi Kristina Brown confirms she and Nick Gordon have transitioned ... from "adopted" brother and sister to husband and wife -- but Whitney Houston's daughter says they got permission from grandma first.

BK and Gordon were doing the newlywed strut in Atlanta Friday ... when a photog asked about their recent nuptials.

The couple was tight lipped on details -- except to hint about a big upcoming ceremony -- but they added ... Cissy Houston OK'd the marriage before they did the deed.

Brown said on her Facebook page in July 2013 that she and Gordon were not ”real brother and sister, nor is he my adoptive brother. My mom never adopted him. In fact, mommy was the one who even said that she knew that we were going to start dating

Brown’s #HappilyMarried announcement on Twitter on Jan. 9 stated “if you didn’t get it the first time.” The rep confirms that the two were actually secretly married in November 2013, when Brown referred to “my hubby” in a tweet.

What do you guys think about this marriage?