Actually, neither form of this sentence sounds right to me.
As you mention, and as Edwin Ashworth points out in his answer, "The suspect, along with his two younger siblings, became the most notorious gangsters in the district" sounds wrong because the parenthetical should not affect the concord.
But "The suspect, along with his two younger siblings, became the most notorious gangster in the district" sounds wrong to me because "the most notorious gangster in the district" can only be a single person. To me, the use of "along with" implies that the younger siblings not only accompanied the suspect, but that they participated in the action described later in the sentence. But logically, it's impossible for them to participate in the action of "becoming the most notorious gangster in the district"... it sounds to me like you're saying they became the same person as their brother.
So I don't think this kind of sentence sounds good with either a plural or a singular noun. I only find it natural if the verb phrase following the parenthetical doesn't specify the number; something like: "The suspect, along with his two younger siblings, embarked on a life of crime."
Note: For some reason, "The suspect, just like his two younger siblings, became a notorious gangster" sounds completely fine to me. But "along with" and "just like" are two different phrases. I'm not sure why there's a difference for me, but I would guess it is because "along with" implies a single action carried out by a single group, while "just like" implies separate actions carried out individually by different people.