Dear Koy,
You should tell your brother what you saw and what explanation his girlfriend gave you. Then stand back and give them time to talk to each other. Try to be impartial and let them discuss this issue together. It's their relationship and sometimes interfering little brothers can end up being given the cold shoulder from both sides. Let your big brother and his girlfriend deal with it but just let him know you are there if he needs someone to talk to.
Good luck,
Annie