Since nobody has done the dumping yet.. If they both love each other (the girl and the younger brother) the girl should be able to make it clear to the elder brother that she belongs to the younger (this is if the girl truly loves the younger brother)

The older is wrong by trying to snatch his own younger brothers wife to be its inhumane. I think this issue should be made known to their father and family relations(if it gets out of infamily solution) before it gets out of hand. (This is if the two lovers are willing to fight for their love)