The problem comes down to what is fair? Your parents may be trying to do what they think is fair, but defining it differently.
Since I'm a lawyer, I'll give you a law-school-like hypothetical, a/k/a my situation: I am my mom's only child; my stepdad has two from a prior marriage. They were married from the time I was 9, so he was truly a second dad; they always kept their finances separate. He died a few years ago, so now the Q is what my mom should do in her will. Of the three kids: I am a lawyer, married to another professional, so we are stable, reliable, etc. Stepsister is stable, social worker, very good person (better than I am), married to good guy with low-paying career. Stepbro is a good person who has suffered from depression/anxiety and has had trouble maintaining steady employment; he previously lived with my mom and stepdad for a while, now lives in his mom's basement. They both received more support growing up (e.g., cars, college loans paid off) due to fundamental differences between my stepdad and mom over what parents should pay for; probably due to those same differences, she saved more than he did. I live very close to my mom and will be the one to take care of "stuff" as she gets older; they live on the other side of the continent. Everyone loves each other and wants to do right by each other.
So what is "fair" here? Options:
1. I get half, they each get 25% (half to "her" side, half to "his"; assuming she comingles his $ with hers after his death and we can't tease out precise dollars).
2. They get all his money (shared 50/50), I get all hers (assuming she keeps his funds separate and lives off of hers)
3. We each get 1/3.
4. Stepbro gets sufficient $ in trust to ensure he has a safe place to live for the rest of his life; the rest is split 50/50 between me and stepsis.
5. Stepbro gets sufficient $ in trust to ensure he has a safe place to live for the rest of his life; Stepsis gets more of what's left so she can enjoy some of the fun stuff I already get to; I get less.
6. Stepbro is written out of the will as a leech and a loser. Stepsis and I split it 50/50.
7. Stepbro is written out of the will as a leech and because he has already received far more economic outpatient care than anyone else. Stepsis and I split the rest 40/60 or 25/75 because I provided more support.
8. We start from 1/3 shares, but I get a little more reflecting the benefit of the paid-off loans, cars, etc. my siblings received and the burden of being the "close" kid.
9. Any of 83 other variations on the above.
I actually don't think there is any "right" answer to this question -- my point is that any of these could be "right" depending on how you look at the facts. So I don't think the right question is whether the parents are being "fair" -- it's what is their definition of "fair"? Talk to them, figure that out. IME, much of the "unfairness" relates to different expectations, i.e., parents giving more $ to one kid because they worry about them ending up out on the street (a/k/a desire to protect vulnerable child outweighs desire to teach kid to stand on feet), which is interpreted by more successful child as punishment for success.
(FWIW, my mom is veering toward "you should get all *my* money," because they always kept separate money; I am pushing for an even 1/3 split, because my stepbro will absolutely see that as a final, damaging rejection. In the end, I am executor, so she can do whatever she wants and I figure I'll fix it once she's gone.).
In the end, this issue is what you make of it. You get to choose whether you let this divide your family; you get to choose whether you let this affect your or your kids' relationship with your parents/their grandparents; you get to choose to see this as the final arbiter of your worth to your parents/your parents' love for you, or as a difference in what different, equally-reasonable and equally-flawed people see as "fair." Their decision doesn't define you; how you choose to respond to it does. When I was a teenager, my grandma remarried a year or two before her death. My whole life, I thought she had left me $3K in the will, which I used to pay my first year law school tuition. Turns out, she wrote me out of the will in favor of her new husband, and my mom paid that money out of her own inheritance. But my mom never told me, because she didn't want it to affect how I thought of my grandma. OTOH, my grandpa was one of three brothers, and the only one who didn't want to stay on the farm. So his parents gave the farm to two brothers, and cash that they thought was 1/3 the value of the farm to my grandpa. But by the time they died, the farm was worth probably 100x my grandpa's share, and he never spoke to his brothers again.
It's up to you whether you want to be like my mom or my grandpa.