I'd first try to talk with them to understand the reason behind the unequal inheritance and let them know that you are hurt by their decision.
I did that, and have been SO GLAD. It was hardhardhard for me to do, to speak and ask honestly, directly, kindly, calmly...with full presence and receptivity to whatever answer they gave.
It took me, I think, two years (!) to get up the oomph/courage/strength to ask, to bring it up.
The conversation happened in three stages, each many many months apart.
At points I was so devastated.
Alone, I cried and cried. Grieved. Saw our shared history in a new light.
The internal shifting was HARD.
Waves of pain.
But one day, a tiny bit of light sparkled.
I began to see things I hadn't before.
How tired my parents were, of trying to figure stuff out.
How profoundly and explicitly one sibling was manipulating them, and how far back that behaviour went.
How there was a multi-person system of dysfunction.
My mum's belief that men need care and women don't.
A comprehensive "forgetting" of history.
That it was nothing personal, not a reflection on me or my parents' love for me.
Within a few days of that glimmer of light—a terrifying and physically painful "seeing"—I had realized what was what, and what decisions I needed to make.
Now, I still don't love that my parents decided to gift one sibling over others.
And that's okay. It's okay that I don't agree with it, support it, endorse it, or love it.
But, I can absolutely accept that this is the decision they made, for the reasons they made it.
It is easy now to not let their decision change my relationship with them.
At the same time, I'm more aware now of my situation, of who is and isn't "there for" me, and in what ways, and I feel freed to make new decisions according to that new knowledge.
Visits are easy, as there is nothing unsaid, no excess pain, nothing tucked away.
This is, I believe, one of the most important gifts we can give ourselves (and our parents) before they pass.
I had said all of what was true for me, my parents heard it to the best degree they could, and they made their decisions with awareness of my views and feelings. I needed that more than I needed equality in generosity/gifting.
I genuinely believe my parents—both of whom went through crazy life stuff and did freakin' awesome with their starting points—did everything they could manage to do. That there was a limit to what they could see or take on or navigate or deal with, well... Once I saw what I saw, I felt no need for either of them to be more, do more, see more. Physically, psychologically, emotionally, and materially they'd performed miracles. There is a limit to the miracles any of us can perform; it's okay for a parent to reach a natural limit. When that light was shed for me, I found all their accomplishments to be enough, more than enough.
Now, several sibling relationships have been affected by my parents' decision to gift one and not others. And that's okay too.
These relationships are not affected "because: money". They're affected because of witnessing the stuff that resulted in the unequal gifting: manipulation, guilt-tripping, severe enabling, codependency, head-in-sand, dishonesty, etc. That kind of stuff often results in reduced respect, regardless of the situation.
But, me and my parents? Awesome Sauce!