Luigi is the younger brother. They are also twins. Iwata calls them twins in this nintendo direct.
Also, on page 5 of the Yoshi's Island Instruction Manual
, Mario and Luigi are referred to as twins. This makes sense, since they were both babies being delivered at the same time.
When official sources claim Luigi is Mario's brother, it's true; he is the younger of the two twins. Probably by a few minutes or hours. They are probably also fraternal twins, since they don't look identical.
There is no evidence indicating they are not twins. There are several official sources indicating they are twins. The answer is pretty obvious.