Ignorance is bliss

Brother told us last night that he has a girlfriend.  He told us that she is in his class and that she has a sister who is her twin.  We pried enough to find out that, though she is indeed his girlfriend - "For sure," Brother said - she may or may not be aware of this status.  I also asked how he can know which one of the twins is his girlfriend.  His response? "They have different backpacks."