@SpiceMelange Oh it is 100% not being stupid. Sibling relationships are the most enduring and can be some of the most infuriating even when you are grown up. The relationships my siblings and I have shaped our lives in one way or another.
My DB has a son who just seems to be the golden child, beloved by all and yet has tantrums and can be quite mean to other kids (saying to ours that he doesn't want to play with them etc) yet he is just 'being silly' whereas ours are sort of pitied and thought of as naughty or 'well what would you expect, you know, they are adopted/being brought up by two women/living down south/taken out every weekend to do enriching activities/allowed to choose their own clothes'. Maddening, but all exacerbated by my own feelings about my siblings who can do no wrong because they conformed to what my parents wanted them to be and do whereas I scarpered and flew the nest asap.
I don't know about confronting them. It depends on your family vibe. I've tried a few times and have been told that I just "come home and stir the pot then leave" when I've tried to have honest conversations about stuff. Plus, few of my family members have time for nuance or subtlety, and can be as pig headed as I am when I comes to believing that they are right, so arguing the toss with them will just exhaust me and ruin what might be a nice time.
That being said, I do find it gratifying to have some way of showing how independent/successful we are in comparison to everybody else's glowing school records and gushing about how polite DN is. On holiday, we plan in seperate family time that is just the four of us and make it something really cool or different that ours will be really good at (like rock climbing, going to an adventurous park or even just seeing a film) and then just sort of drop it in- oh we are going to do xyz today, does anybody want to come with us? That way we can set the agenda for at least part of it so our kids are overtly successful, especially if they are not on their home turf like their cousins (everyone always forgets when they visit, they have come off the back of like a 5 hour car journey into a less familiar enviroment and the expectations are still the same as the kids who are just chilling at home.)
It helps me to feel like I'm not just waiting on them to decide to to activities that I know my kids can't be successful at. I also get a bit petty. My family place a high value on sitting and eating a full plate of dinner, and disapprove greatly that I don't force my kids to do it. But it's not the same for my kids, they just can't do that (yet), but my family insist on going to several high stakes meals out in pubs or big dinners at my parents house. So if they have annoyed me, I pettily lean into being as passive aggressively obnoxiously sweet to my children as I can and make very loud comments about how "of course it's ok that you can go to the play area before everyone else is finished darling" and revel in my parents shocked faces that I would permit my child to do something so outrageous as get down from the table early (see also: using tablets at the table, only eating chips, not sitting down to eat, etc). I find a perverse delight in living up to all their expectations of how they think we parent our child if they can't be generous or kind when we make so much effort to come to these things with them. I would never normally allow my child to go play in the play area during dinner, but then again I would never normally make them sit down for over an hour with only adult conversation to keep them entertained, so to me all bets are off in that situation.
If your partner is supportive , you could go one step further and play bullshit bingo, seeing who manages to predict with accuracy what will be said and by whom. Preempting things is sometimes the best way to take the sting out of it. If you know someone is going to tut or roll their eyes, you could make a bet on how many times, or promise yourself a malteser for each eye roll in a dinner. Even better, try and predict the most outrageous thing your DS will do and who will be most offended. Bonus points for if they literally clutch their pearls.
If I am honest, you seem to be on the right lines that part of the problem is that it all seems a bit tied up in your own feelings about being second best, and that's all tied up in your own self esteem and your relationship with your family. If you had a tonne of self confidence, their opinions wouldn't matter. If you felt at ease and secure in your relationships with them, you probably would feel less compared and less judged. I know that is what triggers my reactions to my family. You can't change either of those things quickly (as I well know!) but you can sort of fake it so that you feel like you have at least come out of some of it on top, rather than it feeling like a slog of negative comparisons.
However, you also don't have to play their games of success and you don't have to tell them anything. My kids have a whip sharp sense of humour and do stupid dances and quote Bluey ad infinitum when they are chilling at home with me, but just can't relax enough to do that around my parents. But I like to know that as my little secret thing that I know about my kids and my parents don't get to know. I have found so much solace in building my understanding of our family unit as something only we get to experience, and that everyone else is missing out on. Can you make a little list of things that your son only does when he is with you and relaxed and have it on your phone to look at? Or a list of funny things he said, or photos of him being super cool and cute for ready access when you feel a bit compared?
And also, I keep reminding myself that I may be bloody knackered, but I'm parenting our kids better than my siblings or my parents ever could have done even whilst I'm functioning on 3 hours sleep.
I think counselling will help if you want to talk things through, and it is always helpful to have someone else be able to say the gnarly parts of your feelings to without judgement.
In the meantime, I hope the wine helped, and that the holiday was bearable!