Anxious time, what to do?

Do what you heart tells you.

Don't hesitate, 'welfare' needs prioritise and I can't imagine anyone criticising you for telling your brother to come and stay with you.
I'm not sure about Scottish prisons but in England there would be both a Probation Officer and Wing Governor in the prison, with responsibility for welfare issues. In your brother's place, I'd telephone and follow that up with a letter expressing my concerns.

Your poor brother. What an awful situation for him. If he's physically well why not let him come for a wee break. It would give him a chance to have a think about what to do and to talk it over with you.
He must be dreading the possibility of his son being released and wanting to come home to him. He'll also be feeling guilty that he doesn't want him home. I'm sure he'll have good reason not to want this. Just because he's his son doesn't necessarily make him a pleasant or even safe person to live with. The poor man. Literally, give him a break!

One other household.

The son is in her bubble, Hetty stated you cannot be in two bubbles.
I thought the poster made it quite clear her son was in her bubble 30 miles away. She is tier 3 and you can only mix with one household. The poster asked for advice on what she should do.

Hetty and rafia I think you might have the wrong end of the stick the poster doesn’t have a son involved In this situation

Your poor brother sounds very troubled so surely some flexibility is in order? I know rules are rules and all that but let's show some humanity. Let him stay as for long as he needs.

Let your brother stay, his welfare seems to be a great concern. Also I would not ask your son to stay away, I would talk to him and see what he thinks. As someone said upthread the rules are not set in stone.
Also if your Brothers son is dangerous, your brothers safety comes first, if he feels stressed or in danger, he should not let his son stay with him.

Yes there are exemptions for emergencies and care for a vulnerable person.

Absolutely let your poor brother stay with you, I would do the same in a heartbeat. I hope things improve for your family.

You can't be in two different bubbles, so need to tell your son to stay away.

I agree with the other posters.
A worrying situation for your brother, but a rotten time for your nephew too, in prison and having mental health problems.Where and who can he turn to if not his own Father.

Yours and his mental health is important so I would say definitely let him come to stay and I hope the situation improves for you all
?

if you do not have a bubble, and he doesn't either, then become a bubble and whatever way you go, you are allowed to share a household. There is no limit on how far apart bubbles are. We are in a bubble wth DD, who lives nearly 100 miles away. She is with us now, staying for a long weekend.

Let your brother come to stay with you. The rules are not set in stone. His needs are greater.

I'm sorry your brother and his son are having a difficult time.

As far as I'm aware, this would count as a welfare issue throughout the UK and it would be in order for you to allow your brother to stay, if you think his mental health is at stake.

I live alone, bubbled with my son 30 miles away who I see infrequently.