Just finished watching Something very bad is going to happen and it confused me very much

So first things first, so the curse was spread to Rachel’s bloodline which is the Harkin’s but I don’t get it why Rachel’s mother had to die because she wouldn’t have any knowledge about the curse if it was in her father’s bloodline and how come she was told by her mother that she would die if she got married.

And when it spread to Nicky’s bloodline how come his mother die and not his father, so does only one of the couple will die?