Shriver appeared on the Today show on Tuesday, where she was asked about the happy news. Pratt, 39, announced his engagement to Schwarzenegger, 29, on Jan. 13 with a sweet Instagram post of him kissing her on the head while she gazes at her new ring.
"She's super happy and gave me super strict instructions not to talk about it," Shriver said. " ... It's not my moment."
"She wants to have her moment," she added, before joking, "Good luck."
Shriver previously publicly congratulated Pratt and Schwarzenegger on their engagement with an Instagram comment.
"Congratulations to two loving, kind, caring, thoughtful, spiritual people," Shriver wrote. "We are so happy for you. You are blessed, as is everyone who knows you. Yes, let's go."
Clearly, the couple has the blessings of their loved ones, including Pratt's ex-wife, 42-year-old Anna Faris. The actress recently revealed Pratt texted her after he proposed during her Anna Faris Is Unqualified podcast.
"Chris texted me this morning and he was like, 'I proposed to Katherine last night,'" Faris shared. "And I was like, 'Ahh, that's amazing.' I texted him back like, 'I just wanted to remind you I'm an ordained minister.'"
"I'm so happy for them," she added. "I knew that it was gonna happen and I love her and I love him and I'm just so happy that they found each other."