The Isley Brothers started their career in the early 1950s by performing gospel songs in local churches in Cincinnati, Ohio. At the time the group consisted of Ronald Isley (b. 21 May 1941), Rudolph Isley (b. 1 April 1941), O'Kelly Isley Jr. (b. 25 December 1937 d. 31 March 1986) and Vernon Isley (b. 1942(?) d. 24 September 1954).

The Isley Brothers  was originally a quartet but sadly Vernon, at the time the youngest of the brothers, died in 1954. 1

The Cincinnati Post Friday 24 September 1954:

"Death Brings Demands For Safeguards
Vernon Isley, 11, Struck by Truck As He Rides Bike

Vernon Isley, 11, tenor in the Isley Brothers' Quartet, was killed Friday as he rode his bicycle down Cooper Road to Blue Ash school.

The road where Vernon was killed is part of a detour that carries heavy traffic which formerly used Montgomery road, and the youngster's death brought an immediate demand to the state for measures to protect children of three schools who must walk along the road to and from school.
Vernon and his brother, Ronald, 13, lead in the Isley quartet, had just ridden double to Sycamore High School. It was Vernon's turn to take the bike. "Be careful, watch the traffic," were Ronald's last words to his younger brother.
The three older Isley brothers are out for football at Sycamore High School.

The family moved to a new home at 9279 Lewis avenue, Blue Ash, Sep. 1. For nine years they lived in Lincoln Heights.

Vernon's father, O'Kelly Isley, works at the new Veterans Administration Hospital as an attendant."

10 Oct 1954, Sun The Cincinnati Enquirer (Cincinnati, Ohio) Newspapers.com

The Cincinnati Enquirer Saturday 25 September 1954:

Killed On Bike
Tenor Of Isley Brothers Quartet Thrown Against Truck In Blue Ash

Vernon Isley, 11, Blue Ash, who sang with three teen-age brothers in the Isley Brothers Quartet, was killed yesterday when he was thrown from his bicycle and landed head first against the steel bed of a truck at 4923 Cooper Rd., Blue Ash. On his way to school, the boy was pedaling on a gravel path beside the road when his bike hit an obstruction and went out of control. Hurled against the truck, the boy suffered a broken neck.
Member of a widely known entertainment group, Vernon sang tenor in the quartet that was trained and accompanied by his mother, Sallye Isley. Other members of the quartet were Ronald, 13, singing leader; Rudolph, 15, baritone, and O'Kelly Isley Jr., 16, bass. The three older brothers attend Sycamore High School."

In 1956 or 1957 Ronald, Rudolph & O'Kelly re-located to New York City and started their career as recording artists.

1 Vernon Isley's date of death is routinely given as 1955 by all available sources. This is clearly incorrect as there were many contemporary news reports about his passing leaving no doubt that he died on the 24th of September in 1954. Even the official Isley Brothers website says he was 13 at the time of the accident which is not correct as the "Card Of Thanks" from the family published in The Cincinnati Enquirer Sunday 10 October 1954 issue states "age 11, who passed away September 24, 1954."

I haven't, however, been able to confirm his date of birth from any reliable source. It's usually given as 1942 which may be correct since Vernon was 11 years old at the time of his death in 1954.