We are sorry to learn of your fiancé's cousin's death. As if your lives weren't hard enough -- coping with the grief -- now you and your fiancé both are dreaming about death! Your recent dreams would cause anyone to wake up confused!
Your first dream shows a clear concern, on your behalf, that you might suddenly lose your fiancé. After all, Justin and his cousin are related. Is it possible that Justin could have the same, hidden birth defect that his cousin did, that would cause him to die suddenly? This fear is transparently reflected in your dream.
Justin's dream, on the other hand, is more curious. Why would he dream of his own death and of you being the one who "pulls the trigger?" The answer, curious as it sounds, is that Justin most likely has experienced a small "death" of his own recently. When we lose a close friend or relative (especially someone who is the same age as we are), it causes us to become aware of our own mortality. When a close friend dies suddenly, we realize it could just as easily have been us.
The significance of you pulling the trigger most likely is that you currently are witnessing Justin's reaction to his cousin's death. Your ability (along with Justin's parents') to see Justin at the funeral shows that, in his time of loss, he still feels very close to you and his parents. We offer our condolences in your time of grieving.