Yes, they are brother and sister. Maggie is older by 3 years being born in '77 while Jake was born in '80. You can watch them on screen together in 2001's, Donnie Darko.
Yes, Maggie Gyllenhaal and Jake Gyllenhall are brother and sister. Maggie is the older sibling by roughly three years. Incidentally, their father is also a director and their mother is a screenwriter.