Soooo sorry to hear about your cousin. How horrific. She must have been in a very dark place.

AND, if your mom is in fact a narcissist, of course she will somehow find a way to make this issue about herself. And so she has.

It's really no one's business, really... it's a very private matter and only immediate family members need to know, at first at least. When I've had similar troubles, we did not share it beyond the immediate family. It's your mom's sister's right to share only when she feels comfortable doing so. IMHO.