Whitney Houston's Daughter "I Got Thumbs Up to Marry My 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.

And then they made out. Gross.

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