No sibling rivalry here.

Millie Bobby Brown, 18, and Henry Cavill, 39, play a brother-sister duo in “Enola Holmes 2,” out Friday on Netflix. Brown had no problem forging a bond with the “Man of Steel” star despite their age difference.

“With Henry, it feels like a real adult relationship. Like a really good friendship, a really healthy one. One that we have terms and conditions,” Brown told Deadline.

“I know Henry. He has terms and conditions with me. I’m not allowed to ask about his personal life. It’s like, ‘Millie, shut up. No.’ And I’m like, ‘Understood.’”

It’s a different type of relationship than the ones she shares with her “Stranger Things” co-stars, including Caleb McLaughlin, Finn Wolfhard, Gaten Matarazzo and Noah Schnapp.

Brown and Schnapp have a pact that they will get married if they’re 40 years old and single. Brown is dating Jon Bon Jovi’s 20-year-old son, Jake Bongiovi.

“With the ‘Stranger Things’ kids, it’s different,” Brown said about her famous pals. “There’s no boundaries because it’s like we’re all siblings. But with Henry, he’s very strict with me, which I appreciate.”

Brown and Cavill fulfilled their professional duties last week while posing for cameras at the “Enola Holmes 2” world premiere red carpet.