Novichok victim Charlie Rowley has been left "looking like a skeleton" after being poisoned by the Russian nerve agent.
Matthew Rowley said his brother had lost a lot of weight after coming into contact with the chemical weapon in southern England nearly two weeks ago.
He said he was shocked by what he saw when he visited his brother in the intensive care unit at Salisbury District Hospital in Wiltshire.
Charlie Rowley has since regained consciousness and is able to speak.
"It's quite shocking to see him because he's not the Charles I know.
"But he is awake and he is talking, making sense - but he is like a skeleton at the moment.
"He is eating now, solid foods, as where he was on liquids for nine days, hence him losing weight.
"He couldn't really keep his head off the pillow, but we managed to have a bit of a laugh together.
"I've never seen so many machines. There's one, two, three, four, five, must have been six machines and he had a lot of tubes coming out of him, a hell of a lot, and he was connected to another set of monitors this side.
"He's still in intensive care. The nurse said he will be here for some time because it's still critical condition."
The 45-year-old remains seriously ill, but after regaining consciousness on Tuesday he has been able to speak to investigators hunting the source of the Novichok.
It is understood that Mr Rowley has been informed that his partner, Dawn Sturgess (44), has died after the pair collapsed at his home in Amesbury, 11km north-east of Salisbury, on June 30 after they were exposed to the nerve agent.
Public Health England said that the overall risk to the public from the nerve agent was low, but advised everyone in the area not to pick up "any foreign object which could contain liquid or gel, in the interests of their own safety".
Metropolitan Police Assistant Commissioner Neil Basu said it is likely the latest poisoning is linked to the attempted murders of former Russian spy Sergei Skripal and his daughter Yulia in Salisbury in March.
He conceded that the authorities had not been looking for anything containing Novichok until the latest incident.
Novichok can remain active for 50 years if left undisturbed in a sealed container.
Mr Basu, who leads the UK's counter-terrorism policing, admitted that investigators have "no idea" what the container looks like.