if youve heard this, then dont spoil it for others, please.

A woman attends her mothers funeral. She meets a man there, and they go on a date. He is very sensitive and comforting, and it turns out she really likes this man. they see each other for about 2 weeks, then suddenly he disappears. She doesn't know where he went. 2 more weeks go by, and this woman kills her sister. Why did she kill her sister?