I have some real jerks in the inlaw department too. I've found that keeping my distance works best for me. I've unfriended just about everyone on Facebook and keep to my own.
Thankfully, they leave me alone too.
I can see them, easily, REALLY EASILY, thinking and saying the words you received from you SIL.
The truth always comes out in the end. They may refuse to see it, but that doesn't change the truth.
You're a great lady and you've been so strong for your boys. Don't lose heart now. Not over this. This is just white noise from people who either refuse or haven't gotten the truth. Either way, it's not your problem, it's theirs. Let them drown in the waves your H is making.
Love to you,
Heather
"You know, it's times like these when I realize what a superhero I am." Tony Stark/Iron Man
“Focus on what you can do, then do it with all your heart.” Lois Wilson