Let brotherly love continue.
Brotherly love - a distinct manifestation of "charity" (2 Peter 1:7). The church of Jerusalem, to which in part this letter was addressed, was distinguished by this grace (Acts 2:45; Acts 4:34-35; Acts 11:29-30: cf. Hebrews 6:10; Hebrews 10:32-34; Hebrews 12:12-13).
Continue - charity will itself continue. See that it continue with you (1 Corinthians 13:8).