The love of one's neighbor, though a universal law, must vary in its practical obligation, according as human relations vary.
When these are so intimate as to become similar to natural brotherhood, the law is termed the law of fraternal charity.
The matter remains the same; brother aids brother to attain salvation.
The obligation is stricter; the exercise, more frequent, particular, and intense.
The climax is reached in religious life, where all are supremely brethren in Christ.
New Catholic Dictionary