Hm. I'm in two minds over this.
Of course it was a dreadful, dreadful thing to do and must have caused you massive hurt, and as others have said, she's totally misdirected her blame and anger, which should really be aimed at your mother.
But... Having been the scapegoat child myself, for whom absolutely nothing went right for a very long time, I can't help but feel a shred of sympathy for her. And while I know you recognise the fucked up-ness of your family dynamics, I wonder how far you can really grasp just how much damage it's done to her, and how hopeless her life seems/actually is?
You talk about some things being her choice - but do you realise how much your choices can be limited by emotional abuse in childhood? It's hard to "choose" a healthy relationship for example when you've been groomed by a parent to be attracted to/comfortable with emotional abuse.
And it's hard to "choose" a well paying, decent career when your self esteem is in the toilet. It's even hard just to make positive heathy choices of any kind for yourself when you've been taught from earliest infancy by the person you loved and trusted and depended on the most and who was supposed to care for you that you're worthless and don't deserve the same love your sister does. It sounds like your mother did a really good job of sabotaging your sister's life adult life before it had even begun.
Added to that, the two things you acknowledge she has absolutely no choice over - infertility and not being loved by her mother - are two incredibly painful, difficult things for anyone to deal with even if they have everything else in place. Which she doesn't.
The thing is, I think you might be sending out some pretty confusing mixed messages to her. On the one hand, you love her and recognise that she's been through some real shit. On the other hand, I'm not sure you really recognise how much all this has affected her, I sense some kind of cognitive dissonance in the way you talk about her. As if you're pretending somehow that you two are on a level playing field, and that there is real equality in your relationship.
But there isn't. You're the princess and she's the pauper. There's you, with your happy marriage, DH with the 6 figure salary, two wonderful children and the love of your mother.
And there's her with her crap partner, no money, and her dreams of being a mother not coming true, maybe never coming true. And she still doesn't have the love of your mother.
Seriously, if you were the "pauper" in this scenario, would you find it easy to be so involved in the "princess"s life? How much strength do you think it must take to be so close to someone who's got everything you want but haven't got? How much must it grind away at your on a daily basis? And don't forget, she's had this all her life, it started when you were children.
Lastly, how do you handle the relationship with your parents now? Do you take her side? Have you ever told your mother that she was/is wrong to favour you over your sister? Would you take that risk for her? You have, after all, benefited from your mother's favouritism, even though obviously you didn't cause or choose it, and your sister has been correspondingly damaged by it. Would you be prepared to do something to even the balance? I don't mean like giving her money because that's just a sticking plaster, I mean like actually rocking the boat of the whole family dynamic. Because as the privileged golden child, you have the power to do that, whereas she doesn't.
I can see this situation must be very, very hard for you too. I've been there with the sleepless nights and the lack of holidays and the exhaustion that is parenting, and it's only reasonable that you occasionally want to vent about all that to someone you're very close to. I can see how it would be so hard to have to try and censor yourself all the time.
But I've also been very, very close to where your sister is now, and believe me, to hear someone who in your eyes has the perfect life moaning about it to you when they know how little you've got - OMG, it galls. (Btw am not implying that you go round constantly moaning about your life to your sister, just taking it from that one remark about the holiday; I am sure you don't constantly rub her nose in it! But just being near you would be like having her nose rubbed in it, tbh, and that is why I can understand her having a massive reaction to that one remark.)
Anyway... This touched a bit of a nerve for me...! Bottom line, I think both of you need to direct your anger at your mother (and possibly enabling father?) for creating this dynamic in the first place and causing this divide between you and the sister you love so much. And ask yourself this: would you have the life you have now if your mother had adored your sister and not you?