I would not contact the cousin.

They did not notify you when about her death etc. Although painful, I don't see how reaching out to know what happened, or satisfying this curiosity, would be beneficial to you. Quite the opposite. I see it as opening the door to further aggravation.

The person who was nice to you is gone. I would leave it at that. Even if you were to know what happened, it would not change a thing - she will remain gone.