JAKE Gyllenhaal has to be himself around his sister Maggie — because she always knows when he’s being fake!

The Brokeback Mountain actor says his older sibling knows him inside out.

“If you have a real relationship with someone in real life they will call you out on it if you are being fake,” he dished. “I have worked with my sister in movies before and I can’t pull anything with her.

“She knows when I’m faking and when I’m not. She has caught me out before — in real life too. That is all my sister does! That is her primary job in my life.”