Sadie Bomar apparently told a US website: "He called me on Christmas Day and told me he's engaged," adding that her superstar grandson asked her not to reveal the secret "until after the holidays".
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She says: "The family loves Jessie … she's a very sweet girl."
The proposal apparently came when the couple were on a holiday to Montana when Justin presented his sweet girl with a custom-designed ring.
Their five-year romance went through a rocky patch last year when they spent several months apart – supposedly because The Social Network actor was reluctant to commit.
It was clear, though, that Justin still felt deeply about her because shortly afterwards he told Vanity Fair: "She is the single-handedly most significant person in my life. In my 30 years, she is the most special person, okay? ...
"I don't want to say much more, because I have to protect things that are dear to me – for instance, her."