Whitney Houston's daughter engaged in some hardcore PDA -- including a kiss -- with her secret BF yesterday ... a guy who happens to be her pseudo-adopted brother ... and TMZ has the video.
19-year-old Bobbi Kristina was seen in a full-on lip-lock with 22-year-old Nick Gordon during a lunch run in Atlanta Tuesday ... confirming suspicions the two are romantically involved.

Again, Bobbi and Nick share no blood relation ... but we still gotta ask ...