If you are prepared to deal with a solicitor, ask him/her to draft a letter, stating that you are prepared to sign an agreement that both parties should abandon all possible future monetary claims on each other, providing she does the same.
Otherwise, make the offer direct. The wording should be as simple as possible, and should state
"I, Jehoshaphat Snooks, agree not to make any financial demands or claims whatsoever on Jezebel Snooks from now on, providing Jezebel Snooks agrees not to make any such demands or claims on me. " Sign and date it, get her to do the same. One copy each, for the files. After all, it would be in her interests too, to stop you claiming on any inheritance or prize she may receive in the future.