My DGS is 7 and is a very gentle placid little boy. Usually.
Several years ago I was taking the DGC for a walk and DGS (4 at the time) and DGD (7 at the time) were walking along bickering with each other.DGS turned and spat at his sister.
I picked up his little hand and gave it a soft smack.
He was terrified and cried his eyes out and I was terrified and shocked that I had done that.
We sat down on a park bench. I cuddled him until he had stopped crying and then made him apologize to his sister.
I then explained to him that only camels spit and donkeys and horses kick. We then spent the next half hour thinking up things that only animals do and humans don't (or shouldn't do).
Even now, two years later he'll sometimes say. "Only camels spit don 't they Grandma"
I'm horrified that FIL actually pulled DS's pants down to smack him and just as the OP things the the earache might be a mitigating circumstance then maybe FIL had had enought of a 7 year old consistently going back and forward and hitting his sibling.
Maybe it was a spontaneous reaction that he regretted afterwards.
Must say that I never smacked my DC when they were growing up.