The pair were travelling to watch a football match when their car veered off the road and collided with a tree near Victoria Falls in their homeland of Zimbabwe.
A female passenger is also reported to have died in the crash.
Peter was capped 100 times by Zimbabwe, while Adam scored 34 goals for his country and spent several seasons playing in Europe for FC Zurich.
"Peter is now out of danger. He is now conscious and is communicating," Ndlovu's former manager Winston Makamure told the BBC.
"He has internal injuries, head injuries, broken ribs and a broken leg."
Ndlovu played 176 times for Coventry between 1991 and 1997, making 196 appearances in all competitions for the Sky Blues and scoring 43 goals.
He then jmoved to Birmingham, netting 23 times in 106 games, and after a loan spell with Huddersfield, joined Sheffield United.
He played 154 times for the Bramall Lane club, scoring 29 goals, and left them in July 2004 when he returned to African club football.
Coventry said in a statement: "The club is saddened to hear of a tragic road accident in Zimbabwe that has left former striker Peter Ndlovu in a critical condition and claimed the life of his brother Adam.
"Ndlovu achieved iconic status during his six years with the Sky Blues ... Coventry City sends its best wishes and condolences to the Ndlovu family who will be in our thoughts."
Birmingham City also released a statement, which read: "The thoughts of everyone associated with Birmingham City Football Club are with Peter Ndlovu and his family following news of a serious car crash in Zimbabwe.
"Peter was a very popular player during his time at St Andrew's and we hope he pulls through this difficult time."
The Blades added: "Our thoughts are with former player Peter Ndlovu, who has been critically injured in a car accident in his native Zimbabwe."