This is a very interstign question. Without looking at the document, I cannot tell you if it is legally enforceable. At first thought, it isn't. Then I began to think of a few legal scenarios that it would be enforceable.
I wasn't clear who you are in relationship to the uncle and two sisters. was one of them your mother? Is he the only one left? What type of annuity is this? In what state? Why did the wills of the others suffiuce, or said another way, why a agreement document and not a will? Any wills or even intestate estates will trump this document in my humble legal opinion, but there are many other factors including the exact annuity, which could change my opinion. You need to see an attorney in your state that deals in trusts and wills and ask him if this document is enforcable. He will ask you all the same questions I didi, and then you will have an idea where you stand, legally.