I think your wife has been wronged, but it sounds like SIL and FIL are clueless.
It doesn’t sound like FIL is even aware your wife has big issue with your SIL.
From their point of view, your wife clams up for ten months and doesn’t communicate with either FIL or SIL and they are supposed to read your minds??
Your FIL is supposed to referee your wife and SIL??