Does sin or expiation result for someone who ejaculates as a result of lust aroused by embracing his sister, knowing that he is remorseful and repentant, and must he cut off communication with her if her messages arouse longing and brotherly love?

Entering upon a mahram without permission is ill-mannered, especially if she is asleep, for fear of her awrah being exposed. The Prophet, peace and blessings be upon him, was asked about seeking permission to enter upon one's mother, and he commanded it. What you did by having direct contact with your sister until ejaculation is a sin, for Allah has forbidden zina (fornication/adultery) and its مقدمات (preliminaries). The prohibition of zina is even more severe when it occurs between mahrams. What you mentioned about a love relationship between you, beyond the bond of kinship, raises concerns that it might be ishq (passionate love). You must fear Allah and remember His punishment. Do not seclude yourself with her, nor touch her, nor look at her if you fear temptation. You must get married, and she must work towards marrying off your sister and preserving her chastity.