I'm so sorry that you're experiencing this stress, and you have options.
First of all, it's important to understand what an "inheritance" is, legally. It is a gift, and not an obligation. So, if your uncle has known you your entire life, he's going to make his Will the way that he wishes. If this sister is coercing him into altering his Will, that can be addressed through Estate Courts.
If you have not been in contact with this sister, for whatever reason, you are under NO obligation to respond or even communicate with her if her discussions are going to revolve around money.
If it were me, I would let things fall as they will. There will be an Executor named to manage the estate, and that Executor must follow the wishes of the benefactor, your uncle.
*****PLEASE, NOTE: I am not an attorney, nor do I have a license to practice Law in any State. What I am going to type, below, is based upon my personal experiences, only. If you have any questions, it is BEST and ADVISABLE to consult an Estate Attorney about your options.*****
Next, it is vital to understand what will happen with the funds in the Estate. First, all residual financial obligations must, by Law, be satisfied. If there are mortgages, loans, medical expenses (not covered by insurance), outstanding utility bills, and funereal costs, those must all be satisfied, FIRST. After those obligations have been satisfied, then the Estate is distributed only after being "probated" by the Court, unless a "Living Trust" was recorded which prevents the need of going through Probate Court. Of course, this applies to the United States, and specific States have their own Laws with regard to Estate Probates. The UK has similar requirements, as does Canada, though I don't know the specifics of their criteria.
Probate Court determines the validity of all Last Will And Testaments and "find" all possible relatives that might be possible beneficiaries of an Estate.
Finally, if, after all debts have been satisfied, you feel that you are entitled to a portion of the Estate, you have the option of "contesting" the Will, especially if you believe and can prove that this sister interfered or coerced the uncle into altering his Will.
So, a million dollars may sound like a shit-ton of money, but after all is said and done, there isn't going to be a whole lot left to distribute. And, that the sister is literally waiting for this uncle to die so that she can have money is a very unseemly and disturbing thing. You may decide that this uncle's money isn't as important as your emotional well-being. People do insane things when money is involved, so it will have to be your personal decision as to how you approach the whole sad matter.