I think that this thread is slightly different to others that are on gransnet about a dil, in that you said that your son seems quite happy with her and their family[did you mean children, or his ils or both?]. So you really dont want to rock the boat too much.
Which brings you to the relationship between you and your DD and, your dil.
I think Anya has given some good advice for starters. See how her ideas pan out first, and then come back here if they dont work?
My other idea, would have been to record some of what she says to you and your DD.
Bullying is horrible. Low level or otherwise.
Perhaps when you play back a recording in private, you and your DD can listen, and perhaps think that it sounds better than you thought? Or it could be worse. And on no account, play it to other people.
It would be some sort of evidence for you and your DD, but if your son is happy, I dont think playing the recording to him at some point would work in your favour, unless the situation gets quite a lot worse than it is now.