I used to work somewhere where we had a young person with autism come and volunteer- in some ways they were quite capable, and could do even quite complex tasks with a lot of repetition and support, and were mostly happy interacting with people. But they needed an awful lot of prompting, and they needed to be shown things maybe ten times, instead of once. Unfortunately, I think in most work environments, the reality is they wouldn't be employed, because there would be someone who was clearly more capable at interview. I definitely don't think they'd make it through probation, unfortunately.
I also teach students who need a high level of support and prompting in all sorts of ways. They can do things, but they need a lot of help and support- they'll often have some kind of support in school, even if it's not a 1:1 TA at all times. In school, if you lose and forget things all the time, if you trail off in the middle of a task, if you're not able to do certain things safely, adaptations are made for you- and in theory an employer should do this up to a point. But I think, unfortunately, in reality, most won't, especially if the employee isn't really able to advocate for themselves.
I don't know what the answer is. I have seen some coffee chains and similar employ people who appear to have additional needs in roles like cleaning and clearing tables. Could something like this be a possibility? That said, I imagine there are a lot fewer roles available than people who might want them?
Alternatively, would they be able to do something purely online- there are e.g. paid moderator roles on certain websites and similar?
It does sound like your relative needs more support and help than they are getting. Do they currently live independently?