tl;dr version:
Couple, drinking, then fighting.
Woman (being the sweet, honorable creatures they are) texts brother that bf has broken her rib, etc.
Brother dons cape and flies over to bf's place, stabs bf.
Oops, sister was lying. Bf still dead, though, so brother looking at 20 years.
Lying slag looks pitiful, but gets to go home, of course.
"Let's you and him fight"

Gentlemen, step AWAY from the cape.