OP, I get it. I have spent the last 3 decades with a MIL who has told me to my face that I am not blood, not family and a SIL who relishes in and laughs along at her comments.
In my 20's I was confused why they treated me like that. I tried hard to get them to see what a great person I am.
In my 30's I kept the peace for the sake of my DH and DC, their only DGC
In my 40's I distanced myself and dropped the rope resulting in seeing very little of them. This upset and annoyed them and now they hate me even more!
Now in my 50's, when I see them on our LC relationship, I challenge every comment, every snide look, any disrespectful action and my attitude is "bring it on, you will get it back with shovel loads".
Yes, of course they are entitled to see their mum on their own on a spa day etc. etc. However, would you treat your SIL like that? No you wouldn't. We do not treat my brother's wives like that. No way!
When the crunch came, they showed you that you are not really family, you are OK, but you are not blood, just someone who their DB is married to. You are dispensable. The question is what do you do with this information?
The nuclear option is to go NC, screw them, totally drop the rope. Or, set yourself up with decades of heartache being made to feel like the bottom rung on their family ladder.
As a happy medium I suggest an assertive compromise where you say, "Have a great family time at the Spa. We are on holiday too, so we won't be spending our holiday time babysitting and at a soft play. We can do that anytime. We are going to XXX castle/ climb this Monroe/ see some Highland Games with our DC. Enjoy!" The bit that gets me the most in your thread is your SIL's saying they want the cousins to play together as they are family. Who the hell are they to dictate terms when they are off at a spa and excluding people.
I used to do this with my MIL/SIL. I cannot control what they do, or how they treat me, but I can control my reaction to it. So my answer would be that they may be able to exclude me, but they don't get to control my time whilst they are doing it. I am not waiting around waiting for them to throw me a fish.
Also, you have a DH problem. My DH's family has massive politics in it, and my MIL and SIL play games with people. I refuse to entertain them, or be a walk on part in it. I have my own thread about Christmas and how my MIL plays Christmas chicken with my DH and I. This year I am just not inviting them.
I suggest you take a couple of steps back from your in-laws. If you are not family enough to be included in a girls day out, on holiday, you are not family enough to host Christmas in your house/ pick out nice presents for your DH's family/ babysit their kids/ go out of your way for any of them.
Let your DH do it all from now on and in the meantime grow a pair of balls.