Was sir lancelot king Arthur's foster brother?

No. I can't believe you think that. Anyway sorry, it's just I've never heard anyone ask that. Lancelot was fostered by the Lady of the Lake after his father King Ban of Gaul (roughly present day France) died. Arthur was fostered (years earlier) by Sir Ector, a faithful knight of his biological father Uther Pendragon. Arthur had a foster brother named Sir Kay. So actually Sir Kay was Arthur's foster brother. Hope I helped.