The legendary boxer, 72, was too ill to attend the premiere of a film about his life last week – or to take part in any of the production for it.
"I have not been able to talk to my brother about this because he is sick," Rahman, 71, told the Sunday People at a screening of I Am Ali in Hollywood.
"He doesn’t speak too well. But he is proud that we are here for him. He has given this film his blessing."
"He has not seen the film yet, but I am very excited for him to see it," Ali’s 46-year-old daughter, Maryum, also said at the screening.
"He is going to love it. I know he is. He is going to cry, he is going to laugh. He will be very proud."
The documentary focuses on the family life of Ali – the three-time world heavyweight champion famed for his quick wit and lightning speed in the ring.
Director Clare Lewins pieced the feature-length together using Ali’s own audio journals, as well as oral histories from rival boxers, like Mike Tyson and George Foreman.
Muhammad’s son, Ali Jnr, said in January he believed there was "no chance" his father would survive until the end of 2014.
"I just want, hope and pray to God that this awful disease takes my dad sooner rather than later," he said.
"Take him away from all the suffering he’s in."
I Am Ali will be released in cinemas across the UK from 28 November. Muhammad Ali will finally get to see the film himself when he is shown by his daughter Hana, 38, at his home in Arizona at the end of the month.