Some advice for complex family situation

My sister is schizophrenic. Her schizophrenia has been borderline over the years but as she is aging, it seems to be getting worse. I thought she was starting into dementia, but it just looks more like a deeper level of her delusional thinkings. She is not paranoic but mostly has very disordered thinking and behaviour. Often being impulsive. She is also a chronic (of the worst kind) hoarder. She is also carer for her paranoid schizophrenic son. The son is nearly 40 and she has just turned 70.

My sister still has contact with her ex-husband and he tries to help. They are a horrible mismatch but he does try to share some responsibility with this son, along with another son who is also on the spectrum. However, my sister over the years has rushed into four very crazy relationships that never worked out, which is fortunate as none would have been healthy. Now she is forming a new relationship, mostly online, but she has met this man by travelling to see him briefly and they are now talking about marriage. This is the usual pattern for my sister. Her first marriage happened after she had gone out with her boyfriend for 6 weeks and 3 weeks later they were married. Each time she has been in one of these fast moving relationships, she was unable to marry as she hadn't divorced. However, now she is a free agent.

I have not met the boyfriend (he is 72). But I doubt very much that he has any idea what he is buying into. I don't feel that I can say anything. If I did say something to him, he would probably pass this on to my sister. When she is in these early stages of relationships, the man usually only sees a sweet lady and she usually tells them that her family does not believe in her etc.

I am trying to encourage both to just go slow, but I can see the relationship escalating. The guy lives a long way across the country and so I have tried to encourage him to come and stay at our place so that he can see her on her own ground. I am not sure that this will happen.

She has also painted a rosy picture of my nephew who is very high IQ but completely nonfunctional. He too spends much of his daydreaming of great enterprises that he feels sure he can do. Nothing other than sleeping and sitting about and never showering actually happens. But my sister's boyfriend thinks he can care for both my sister and this son. It is a disaster waiting to happen all around. I am concerned for him as much as anything.

I think that this big idea is something my sister has worked out as she is well aware that she is becoming less and less able to care for her son. And now this man will carry them both.

What can I do?