Whitney Houston's Mom -- PISSED Over Bobbi Kristina's Marriage

Bobbi Kristina and her "adopted" brother/new husband SAY Grandma Cissy Houston gave them her blessing to tie the knot ... but we're told that couldn't be further from the truth.

Bobbi Kristina and Nick Gordon are now married, and they proudly told us so. They also claimed they attended a Sweet 16 soiree, and that's where Cissy gave her thumbs up.

But our Houston family sources say they were at the party ... neither BK nor Nick ever asked Cissy for her approval.

In fact we're told Granny Houston was PISSED when she found out the two tied the knot last week ... but realized there's nothing she could do it about it.