DB and SIL came to stay with their 2 DCs 5 and 2.
After they had gone, I noticed my old bear was missing. Looked high and lo but nothing.
Spoke to DB earlier and asked about my bear. Turns out the 5yo had taken a shine to it and asked SIL if she could take it. SIL SAID YES!!!
To make it worse, she only went and bloody lost it on the way home 
I’ve had this bear since I was a baby. It was my most trusted companion and confidant throughout my childhood. DB knows this. But he was laughing whilst being apologetic about the whole thing.
I know I’m overreacting and it was just an accident but I am SO upset.
AIBU to never speak to SIL again? (Obviously I will through gritted teeth but right now I feel like having a full on tantrum).