Some visa types allow child derivatives. The IR-2 (which is what your mom will get, does NOT). So your sibling needs a separate petition, and will have a separate visa category, regardless of his age.
Sounds to me like the immigration lawyer didn’t have all of the pertinent info from you, you explained things poorly, or he’s just incompetent. This is basic family visas 101, no rocket science here.
And it’s not “15-20 years processing time.” There will be nothing done with the petition during that time. The wait is for a visa number to become available, and the reason it takes so long is that billions of people also had that idea and are in the queue ahead of you.
Petitioners die all the time, and in some cases there can be a humanitarian reinstatement. But this is why it will be a good idea, once your mom is an LPR, for you both to petition the brother.