Maria, I am very sorry to hear what happened to Sassy. I also feel the unbearable pain that you are right now. I lost my beloved 16 year old cat Rupert today, a cat i grew up with since i was 6. I did feel guilty too, thinking i could of saved him too if i took him to a vet a week ago. I wish from every fiber of my being that i could have made him last forever. Unfortunately, it is impossible.
I do agree, some people don't know what a pet could mean to a person, cat or dog, or any other kind of pet. There are like another family member to you. Your son probably felt she was an older sister, and she must of felt like your daughter from another species. I feel so much for you and your son. He will understand soon. He will never forget Sassy. Not talking about her does not mean he will forget her. We all need time to process and heal. It's a long road to recovery, but we can make make it. How? As a man once said "Day by Day."
My friend, my brother, Rupert, pre 2001-January 25, 2016 I love you and I miss you.