No, it is based on assessment. If a worker had any doubts, if the 18 yo petitioned, it would go to a judge. Who would make a decision based on the assessment, unless there are extraordinary circumstances, the 6 yo is going to go to older adults in the family, if they want him, or foster care.
I wouldn't want that child as a foster child. It would be all sorts of messed up, because I doubt that the parents would release him for adoption. The Social Worker would probably have to take him to arranged family unity meetings with the parents in prison. And the kid comes back, with all sorts of anger management issues.
Unless a judge ordered the family unity meetings off, based on assessment. That kid would be more drama than it is worth, with potential visits with the 18 yo sister, if she even shows up.
Doing foster care is great, as long as the parents and other family members are out of the picture. The kids are no problem. It is dealing with the other family members who are full of drama.