Late at night, the young man sat alone with the girl in the parlor for a long time.
Literal
Night late [at-に] young man [topic-は] girl [with-と] 2-people-only [with-で] long time-interval parlor [at-に] was-sitting-[ていた].
ふたりきり ('just the two of them') uses the limiting suffix ~きり ('only, alone, nothing but') attached to a number. Creates a private/intimate scene. 客間 ('parlor, guest room') is the formal receiving room in a traditional Japanese home. 長い間 ('for a long time') uses 間 ('interval') as a temporal measure.