"Hendes tro var stærk, da hendes bedstemor døde."
Translation:Her faith was strong, when her grandmother died.
Because tro is the subject of the sentence, I think. Sin is only used to match the subject.
It's because sin-sit-sine cannot be used when it is part of the subject. In this case "her grandmother" is the subject, therefore it cannot be sin bedstemor, even though it is her own grandmother, it has to be hendes bedstemor.