OP, I've read through this thread over the past couple of days and you could be describing the difficulties my DM had with my SIL. When DB got together with her, she just wasn't interested. She declined all invitations to family events with one excuse or another, she made it obvious she didn't want to spend time with any of us, and DM, like you, spent years trying to work out what she could have done (DM is v welcoming to any visitor).
It turned out that there were a number of reasons for this, but primarily, they were just very, very different people. SIL hated school and left at the first opportunity. DM was a teacher, so SIL got it into her head she was being 'looked down on' (she wasn't). SIL's family swear and drink a lot. DM & DF do not swear and they hardly ever drink, even at Christmas. So SIL felt uncomfortable and bored at our family dos. On top of that, I think DM had imagined a relationship where she and SIL would be close and go for coffee regularly, but SIL and she didn't really have enough in common to make this likely. But this in itself was a bit weird because DM never had this with her own MIL.
What DB could never admit to DM was that he actually preferred SIL's family bashes to ours (he told me this when we chatted about things). He loves DM, but they are very, very different, and so he feels much more at home with SIL's family than with ours. So the situation also partly arose because DB didn't want to upset DM by being honest about how he saw SIL's family compared to ours.
I suspect you probably have something similar in why things are as they are between you all, and I suspect that it may well be some form of insecurity/uninterest from your DIL at the root of it. But I also suspect your DS is happy with things as they stand and doesn't necessarily think there's an issue.
If it's any consolation, things have improved massively between SIL & us. DM never forced things with SIL, nor confronted her, and things ticked along for a while in that fashion. Then, SIL and DB separated for a while when DN was v small, and during that time we all continued to be friendly to SIL and remained in contact. It was this which persuaded SIL that actually we were not bad people, looking down on her or any of the other things she'd built up in her head. When they got back together, things had changed and she and DM now get on absolutely fine. They'll never be best buddies, and SIL's family will always get first pick of events, but SIL now comes to our family events and enjoys herself, and is pleasant to everyone. So it can change.
In your position, therefore, I'd do what my DM did. Stop over-analysing and worrying about things, and focus on your DS. Don't push a confrontation, keep the invitations coming, and things may well change by themselves over time anyway. And just send a 'thanks for the explanation' type text, not a PA one or one calling her out on things. Rise above it. It's not easy, I know from seeing my DM do it, but it does work. HTH.