The 'American Horror Story' actress, 22, is set to marry her co-star Evan Peters, 26, after he proposed with a pink, gold and diamond ring over the Christmas period in New York City, and while her stepmother Eliza Roberts and grandmother Lila Garrett are "happy" for her, they admitted they were surprised by the news.
Asked for her reaction, Eliza, who is married to Emma's father Eric Roberts, was captured on video by gossip website TMZ.com saying: "I think I know what he's talking about. I don't know if these guys know."
She then proceeded to tell other family members that Emma is "getting married."
She added: "We're very happy for her. If she's happy, we're happy."
Asked if she saw the engagement coming, Eliza said: "I mean no, not really, but it's very romantic and if she's happy, we're happy."
Her grandmother added: "Of course (we're happy), I didn't know about it. As long as we don't see it going, that's the important thing."
Eliza insisted she had "no idea" if they would be paying for Emma's wedding but dismissed the idea that she is too young to tie the knot.
She said: "Some very lasting marriages have started very young."
Emma, whose aunt is Oscar-winner Julia Roberts, was raised by her mother Kelly Cunningham.
Eliza and Eric, who were previously said to have a "distant relationship" with the 'We're The Millers' star, rushed to Emma's defence after she was arrested in July when she and Evan had a fight in Montreal, Canada which left him with a bloody nose and a bite mark, and her with bruised legs, and insisted the incident was a "misunderstanding" and nobody else's business.