Family in the attic

A family consisting of a boy Matti, his dad, his little sister and their grandma needs to get down from the attic before the roof falls in. The stairs are dark and they have only one flashlight and can't go down the stairs by more than 2 people at a time. They have 18 minutes altogether. For Matti it takes 2 minutes to go up or down the stairs, for dad 3 minutes, for little sister 5 and for grandma 7. How can they make it?