I don't think Jermaine never made it big because of Michael success (and also the fact he's a genius). I think it's simply the fact his songs were good but never original enough, and the fact he doesn't have a strong stage and screen presence. Jermaine was also at his best and most relaxed on stage and screen performing secondary vocals to Michael as part of the Jackson 5.
Just look at the below performance of Jermaine singing Let's Get Serious, he lacks confidence as a solo performer, he's so stiff it's like watching John McCain performing.
 
 
Michael and Jermaine's vocals have never sounded the same even before Jermaine's voice broke. The only thing is Michael and Jermaine's vocal's have always complimented and blended beautifully when singing together. All of the brothers sound different when they sing.
I doubt the Jackson 5 would have made it big, If their had been no Michael. I think the family would have been sports stars without Michael, as they are or were an unbeaten basket ball team in celebrity circles. And before they signed to Motown a professional baseball team wanted to sign Jackie as a player. But let's get real, Michael's has always existed, and it was Michael who gave the Jackson 5 the genius that made them a legendary group. The brothers are talented, but Michael was the main man of the family from the time he was an infant. He is the face of the Jackson 5 and the whole family.
As people I'm sure Michael and his brothers have lot's of simularites. They grew up with each and are obviously brothers. Jermaine has said it hurts him when the media say Michael has abused children, because he says all of the brothers are the same and it's like accusing all of them. Tito has said Michael and Marlon are really close, closer than the pubic know. This thread isn't about distancing Michael from his brothers, it's just saying that his brothers aren't huge success and talents like Michael. They deserve a lot more credit as part of the Jackson 5/ Jacksons than they get, but it is Michael who made the group amazing because he is the amazing talent of the family. I'm sure at one time Michael's success and talent was envied by some (not all) of his brothers, but now they would say Michael was what made the Jackson 5 amazing, just as Michael would say his brothers are a huge part of his legacy as as solo artist because it's the Jackson 5 that stared his career.