Today I texted SS30 and let him know how sad we (my kids and I) are about missing his engagement party. And then I sat on pins and needles, waiting for his reply. And he shocked me. He said that he was sad that we weren't coming but that he "gets it". He said hd knows that the rift in the family is getting worse, rather than better, and that it's very upsetting. (With no hint of blame tossed in my direction!). He said that "for the record" he wants me and my kids to be a part of important events in his life but that I should do what's best for me and my kids. 
This is the first time (in a year!) that anyone in the family has empathized with my situation rather than blaming me and accusing me for ruining their family. And he's the only one who's opinion matters right now, because he's the only family member DH has at the moment. Maybe SS is also starting to realize that there are many layers. I won't get my hope up, but at the moment, I'm relieved.