Stubborn elderly, disabled father

My dad is 82, has severe RA and has had all joints replaced, uses a walker but is thisclose to needing a wheelchair. He lives in another state w/my sister, and she is unable to care for him properly. She also has RA and addiction issues. It's horrible. I am already swamped w/caring for my family, my recently injured son (car wreck), and my elderly MIL who finally moved a mile away (senior housing). I thought I had my dad lined up to come here too -- room at the same senior apt. complex as MIL -- when he told me he's not coming. I guess this is typical. I am so frustrated. My sister is a MESS. They live in squalor -- 500 square feet for both of them, living on his SS. :P It's just so frustrating. I was driving out (from CO to OR) each summer for a week w/my 6 kids to check on them, but I can't do it anymore. I have fibromyalgia myself, and 2 college aged kids, plus 4 littles. I just need more downtime. I am trying to let go -- they are stubborn and insist they are fine. I guess I have to let them do their own thing, huh? GRRRR. :(