Totally justified feelings and reaction. It's awful to be confronted with clear evidence that those you consider your nearest and dearest, don't feel the same way you do.
Most of us encounter this at some point either like this - birthdays etc, or when we're in crisis and need support/help/a friendly ear.
I would try to focus on the positives, if you can.
1 cebrate what wonderful SILs and caring husband you have. My SIL despised me from the day she met me for stealing her big brother and for 20 years went out of her way to make me suffer for it; and after putting up with that and taking it so as not to cause family rifts, we ended up separating! Good SILs are very underrated
2 you know how they see the friendship. You can adjust your behaviour/feelings to a more appropriate level for the type of friendship it is.
3 you can invest less in the friendships as they're not as important as you may have thought, which gives you time, energy and emotional capacity to find other relationships to invest more in
4 Doesn't hurt any less but - you're probably a better friend than they are. It's not about you - it's about them. And deep down you know this.
And so on...
Personally, I would need to say something and so I would. I'd probably write an honest text telling how hurt, disappointed and upset I am and how at least now I know how we stand. And then I'd not send it.
I'd review it an hour or two later and tweak it. And not send it. Then I'd sleep on it, tweak again and decide whether to send or not.
As I've gotten older and wiser (or maybe giving less of a shit!), I tend to not send. The writing it down and getting what I want need to say right is cathartic and enough, most times.
But I would say something short and 'closed' along theines imof
'hi. You missed a lovely special birthday afternoon tea. And you were missed - I was quite upset and hurt you chose not to come. I wanted to tell you how I feel. It's not to start a discussion so please don't.
Speak soon x'
And yes, it will change the dynamic(s) but they've already done that.