it's not clear to me from the letter if Emma was the late brother's child or not? The LW refers to her as Sara's daughter only, but her venom for Sara is so intense that I cannot tell if it's just that or if it is that Emma was the late brother's stepdaughter.

I suspect that Emma is the late brother's stepdaughter, and therefore not his legal heir, thus the LW's conflict. Or if Emma was included in the original trust by the grandparents and afterward cut out?

Also the LW should get some therapy for how angry they still are at Sara. That is abnormal and super toxic.