Spoiler Warning: The following bookmark contains plot information relating to The Crossing and Cities of the Plain.
This remarkable passage won't make much sense taken only in the context of The Crossing. It foreshadows events that will come to pass in Cities of the Plain, the third and final installment of The Border Trilogy.
The dying brother is not Boyd, but John Grady Cole, the lead character from All The Pretty Horses who becomes, in a sense, a spiritual reincarnation of Boyd. History will repeat itself in Cities of the Plain when Billy Parham is once again unable to save the younger man from death.
The girl is the young prostitute John Grady falls in love with.