My mother have got a german passport after marrying my stepfather. Can my brother and I get a german passport if my stepdad adopt us even though we are over 18 and not born in germany.

I would assume you could, if the step-dad (a german citizen) will sponsor you to move there as a family member. I don't think they would give you a german pasport if you don't live there though.

Go to the german immigration website (I assume they must have one, there you can get all the laws for sponsoring a family member to move there)