Emma Roberts -- Family In the Dark Over Engagement

News of Emma Roberts engagement clearly blindsided some of the people closest to her ... kin without a clue.

Emma's stepmom, Eliza Roberts and her stepgrandma, Lila Garrett were at Il Buco in Beverly Hills Tuesday night when we offered our congratulations.

You gotta see their reactions ... because clearly Eliza had kept the engagement from her own mom.

As you know ... before getting engaged Emma was arrested after allegedly beating up her now fiance, her "American Horror Story" co-star Evan Peters.

Lila's reaction is brilliant.