This is exactly what i have been considering doing with my father and his health is also failing. I think it would be a good idea, if you know they have no way of turning it against you in the future. I sent my father a series of letters when i was a teen, ranting about his abuse. He never responded, never apologized or even aknowledged them when i had to face him. I knew he got them as his at the timw girlfriend said he would get so mad and refuse to read them, then she would have to read thenm to him, and supposedly they made him cry. But he never changed, never aknowledged his wrong doings. So remember the letter isnt meant to change them but its more of getting closure in your life. They may never admit or apologize, but you can heal without their aknowledgement just knowing that you made your voice heard