We do not have enough information in 4 posts to give feedback here from OP + drip feed.
If this happened, this is a serious issue.
Of course the MIL will think her husband did not do it.
Generally speaking, not applicable to this post, the victims of sexual harassment and abuse, it takes them years to speak up and take action.
Also, women, when they speak up, are not believed.
As usual, it is a war between the women in the house and the men are quiet as mice.
Why no posters have asked:
1. What are the personalities of FIL and DIL?
Has anybody else raised concerns about DIL/FIL's comments told to other people?
How does FIL/DIL react to other people, apart from DIL/FIL? Is he/she easy going? happy? reserved?
Both questions apply to both
Of course they don't have to be answered in the forum - it is just questioning the echo chamber.
2. Why is not son doing anything about it?
No, losing his kids is not an excuse