"The only pain was in leaving her father, who would certainly miss her, and who, when it came to the point, so little liked her going that he told her to write to him, and almost promised to answer her letter."

What a sweet hint to the special relationship, but it seems it is out of the question that a father would take the trouble of writing to his daughter just for correspodence sake. I just realize that it might be very unusual that Darcy writes such long letters to his sister, obviously without some special reason ...