I can understand where you are coming from, but I think your attitude to this is a bit simplistic op. We know that people aren’t necessarily happier for a start after a certain income point. And as your dsis is now the designated go-to person for everything domestic and child-related, and her husband does all of the earning, she may feel burdened by lack of autonomy, equality and control in her situation, which can lead to frustration, feelings of worthlessness and depression. Constant moaning can be a symptom of depression btw. Everything isn’t always how it looks from the outside,
One of the central themes that married people argue about is the balance of power in the relationship, and in a high earning man + sahm situation, it is common for that balance to be out of whack.
Also, until you have dc, and are in this situation, it’s difficult to know how your personal experience of being a sahm will turn out, as there are no formal rules surrounding your role, and none surrounding your financial “payment” either, so the success of the partnership depends to some extent on how much the husband values the wife, and to a certain extent, on how much she values herself. And on how open and transparent he (or she*) is about finances and to what extent he allows her to make financial decisions in the home. (*Btw, I am using the terms sahm and he/she but this applies the same if it’s the woman who is the main bread-winner too,)
Finally, saying this very sensitively, as you are on a very stressful IVF pathway atm which I hope is successful for you; I would argue that until you have dc, it is pretty impossible to gauge the level of worry and stress that comes with parenting.
Yes,your dsis has massive advantages: being able to buy in domestic support helps immensely, as does a lovely spacious home, as does being able to be at home and change your plans for the day when dc are ill, during school holidays, or when they need you more emotionally, as does being able to go at a child’s pace, particularly during the early years.
And once they are at primary school, there’s a lot of practical faff that comes with it which is stressful, when you have a WOH job. Ditto teen exams and emotional issues later on.
But even with these massive advantages op, your sister’s life isn’t completely stress-free, because the saying is true, that you are only as happy as your unhappiest child. And the worry when you have dc in today’s world, can be pretty crushing.
But I’ve mainly voted YABU because she is your sister, and if your sister doesn’t have your back, then who does? Of course that applies reciprocally but sticking to your main question in the thread title, I would hope that a sister, would look a bit deeper at the emotion behind her words and complaints , and would take her aside and , yes, remind her gently of her privileges, but also initiate a kind non-judgmental conversation about what is going on with her.
It could be that she senses the slightly judgmental attitude that you, and others, hold towards her current role, and therefore bigs up the difficulty of it, as a means of self-protection, or she really is struggling, but until you really talk to her properly sister to sister, and try to understand where she is coming from, in a non-competitive way, you won’t really know for sure. 💐