My 2 cents: As long as a person is physically safe - and it seems like your sister is - I would hold off. If your sister has enough where with all to use words like 'agency' - she might be ok for a bit yet. I think the ones best able to know what's really going on with her are the Humans who are around her every day. I dealt with my father's later years as he slipped into vascular dementia. There came a day when it was obvious to all that he couldn't live on his own, even with home support. You'll know. My father got to the point where he couldn't do his activities of daily living without help. He knew he had a daughter who was nurse, but he thought my name was Dorothy. We weren't in Kansas any longer.