Sorry to pick this one up again but what if the reason for child 1 being able to cope in circumstances when child 2 isnt might be due to personality differences rather than chemistry imbalances or personality differences may cause a different chemistry makeup of that person.In my view you are putting to much emphasis on one aspect and you are looking at it in a linear way. On the other hand, if you are looking at it from the "biology" glasses then why do you get so confrontational with R- appearantly he should be the victim of some inborn imbalance and none of his behaviour would be his free will. (Do you see what I am saying?)
If he is bipolar you shouldnt judge with your "normal" standards but look at him from a biological point of view and see that he cannot help it. And from that point of view his disorder is a coping mechanism and him trying to attract constant attention isnt a maniulation but a means of getting what his psyche needs - looking from biology.