I'd say that where you go from here is away from your never-loving daughter, and let her make the next move. She and her brother have your share of their grandmother's legacy, which MiL wanted you to have, and she has behaved like a sulky four-year-old about a decision which was made, not by you but by her grandmother, and which you have already reversed when there was no need for you to do it.
That is, unless there were things said and done on both sides which you are not telling us about. We don't know whether there were things you said to her that she cannot forgive, as well as the things she said to you. We can't tell whether she believes that you influenced your MiL into including you in the will, or whether she has good reason to believe that. We don't know what jealousies have been simmering for 15 years and have been exposed by this will business.
We only know what we read, and we don't even know whether what we read is truly the right story. So any advice given can only be very general.