Honestly, I understand how you’re feeling, but eventually you’ll be able to see that SIL is entirely isolated in her situation, which is great.
She was asked to leave, your DH is handling her, and your PILS sound as if they’re very happy to take their cue from you.
Therefore, this is a horrendous situation, but it’s all being dealt with in just the right way.
You can’t do anything about her appalling behaviour, but at least everyone else sounds human. Presumably there’s nothing to stop her daughter spending more time with her grandparents if she wants?
I have a SIL like this, and it’s had a huge impact on my whole life, marriage, everything. Mostly because her parents and my DH won’t tackle it. They recognise it, but they just look the other way. Seize the moment, and make sure that it is clear that she will not be able to visit PILS whilst your sons are there. It doesn’t sound as if they’d disagree.