YOu have had a lot of sensible advice.
I would just add my bit. And please bear in mind that I dont know you, or your family, but go by my own personal experience.
I had a very rich uncle. I loved him very much, we were close and good friends. I have many happy childhood memories with him.
One day he came to me and said something along these lines. QS, you know I am old, I have ill health, I am preparing my estate for my death. You know, the way I see it, you are married, you have a house, you are doing well, you and your dh are running your own company, I have no worries about you. I worry about your sister on the other hand, she is a single mum, she has bad health, much less earning potential than you. I am planning to leave her the bulk of my estate. I just thought you should know.
When my dhs grandfather was getting on, he rang my dh and said to him something quite similar. "I am planning on leaving my flat to your youngest cousin, she is the only one who doesnt own her own home, she is just starting university, she needs it the most."
I think this is fair for the following reasons:
People should chose who they want their estate to go to, they very often have their reasons. In many cases they have the best insight into the lives of their children, there may be things in the other childrens lives which are not being discusses, and not common knowledge, and sometimes it is not really "suitable" for the other children to start questioning their motives.
I agree with my uncles decision, he had very valid points. I was not left penniless entirely, but he did the right thing. (He was not to know about our later struggles, but by all accounts and purposes, my sisters struggle has been harder, and her life would have been more shit than imaginable without my uncles inheritance.)
So before you go in all guns blazing, make sure you know your story. And I think Xenias suggestion is good, just make sure you dont try force something through that your parents wouldnt want.