Will it wind up being no sister left behind? Elizabeth
Boyd Holbrook, 32, popped the question to the "Martha Marcy May Marlene" star during a recent trip to Paris, Star reported.
"They've been by each other for a while, taking a lot of couple time," an Us source said of the newly engaged twosome. Another added that the thespians recently moved in together in Brooklyn.
They met in 2012 on the set of the movie "Very Good Girls," Star said.
Meanwhile, Mary-Kate's twin sister Ashley, 27, is dating "Moneyball" director Bennett Miller, 47. Their difference in age is similar to Mary-Kate's with fiance Oliver Sarkozy, who is 44.