Ex DIL is the mother of your beloved grandchildren hence she'll always be a part of your family too. There's absolutely no disrespect on your part and your loyalty towards your your ex DIL (and your GC) is well placed.
It's very unfortunate that DIL 2 is so insecure, but that doesn't give her a pass for wanting to divide your family. Her baseless accusations and downright rudeness create more distance between you and your son. She's not only being rude, disrespectful and disloyal towards you, she's pushing you away by behaving like an artful instigator.
So, she's the one with the problem, not you. You could try to reassure her, but that could backfire. Probably best to carry on as you always have. Ignore her for now and hope that your son eventually sees her for who she is.