I will state my theory, though I would like to ask a few questions. Was she found at your house? Was anyone home? And were there any family disagreements or arguments? My theory of course it may not be correct, is that the sisters were involved, though once again it may not be correct. Of course, this is assuming they were home at the time of her murder. It is possible they faked “not thinking the evidence was important” in order to remove any evidence. The brother was not mentioned until “primary suspect” so I am not sure if it is him or not. Another question is did the sisters have a boyfriend or relationship? If so, it is possible that their boyfriends killed your mother, especially if she disapproved of the relationship. Of course, this isn’t a definite, especially without more answers, this is the most I can do.