Each was the youngest member of a brother singing group

and then went on to a successful solo career.

The Osmonds started recording in 1970, and in 1971 Donny already had solo hits.
The Jackson 5 was formed in 1965, and Michael began recording as a solo act in 1971.
The Williams Brothers began in 1938, and Andy's solo career began in 1953.

Yes, I know Michael Jackson had a younger brother,
but the group I'm referring to is The Jackson Five.

And yes, Donny Osmond also had a younger brother,
but he wasn't in the group when they started recording.