The daughter of the late singer posted a picture of the pair holding hands, showing off what appeared to be wedding rings, with the caption: "'#HappilyMarried So #Inlove if you didn't get it the first time that is."
Bobbi Kristina, who was just 18-years-old when her mother died on February 11 2012, got engaged to family friend Nick in 2012.
She was forced to defend the relationship on Facebook after their engagement was revealed to the world in reality show The Houstons: On Our Own.
She wrote: "I'm tired of hearing people say 'Eww you're engaged to your brother' or 'If Whitney was still alive would we be together' or 'Would she approve of this.'
"Let me clear up something, we aren't even real brother and sister nor is he my adoptive brother. My mom never adopted him.
"In fact, mommy was the one who even said she knew that we were going to start dating."
The pair met when Whitney cared for Nick as a child.
The 20-year-old went on to defend her relationship insisting she wasn't doing anything wrong.
She also called on people not to pass judgement on the couple and their romance without knowing all the facts.
She added: "People need to seriously stop judging my relationship. Pretty sure it's my own decision who I want to be with.
"Yes, my relationship may not be perfect. We will have rough patches, just like every other relationship. And we have had our rough patches.
The pair have had an on-off relationship, with Nick later tweeting he was not engaged or dating Bobbi Kristina.
But in September 16 newlywed Nick tweeted that he had upgraded the ring and added two bands of #blingbling.