Well, hard to even be able to pinpoint what might be happening in this one. After a read through, my initial impression is this is some kind of sick, twisted case of jealously and gaslighting. Does you partner have a history of mental health issues? Jealousy issues?

The way she reacted when you asked if A had died could either be a light hearted reaction because she has no idea what's going on, or it's a very strange reaction that points to the real reason all of this happened. Why did she immediately ask if you hated the cousin?

Finally, is there anyway you can verify she did show you this picture and tell you A was deceased? Did she talk about A's death in front of anyone else?