In Japanese you say onii-sanonii-san is older brother. Nissan (Ni-san) is big brother. However, this is the formal way. For a more relaxed informal way, you delete the san and replace it with a Chan....
You may either say 'boku wa sukotto no ani desu,' ("I'm Scott's older brother") or 'sukotto wa otoutou desu,' ("Scott is my younger brother") These can be written (in the same order):...