Is an apology due for something not done? Are deeds rejected because of abandonment in which we had no hand? And is greeting and sending religious messages sufficient to exit the forbidden abandonment?

The abandonment in the question is from the side of the questioner and her sister, not directed at them, and there is no blame upon them for that. What you mentioned about exchanging greetings, congratulating her on occasions, and sending messages is sufficient to avoid the forbidden abandonment. Indeed, according to most scholars, merely exchanging greetings and the like is sufficient.