Hi, M.. I have a 15 year old son as well. He, well, he and I have gone through a lot of pain and loss. His biological father, my first husband, died when my son was 19 months old. My son is a really great kid. He gets Almost straight A's in school (he has one B). He is polite, and whenever we are around other people, there are usually comments about his manners, and if he is at a friend's house the report is always stellar from the friend's parents. I am remarried now for 6 years, have a 4-year old daughter, and he is very helpful with his sister. I have also been through two serious surgical, life threatening health issues in the past year. I also continue to have health issues fron that same problem as well. My son has not cried one single time. My husband and I have talked and talked to him. We have even gone so far as to point out to him that if I were to not make it through one of my surguries that he would technically be an orphan, since my husband has never adpoted him, and my son even said that he didn't care, and that he just knew that I was going to be OK, and that he just didnt care about what was going on, and to please just leave him alone. We were not fighting at the time, we were all three takling calmly, my son was just talking matter-of-factly (at least he said please! LOL). I was never upset or hurt or offended by my son's actions or words, even though my husband was outraged and very hurt by this, because my will names my husband as the custodian of my son in the event of my demise. And also because, obviously, my husband was scared to death for me as I was about to undergo a second life-threatening surgical procedure in less than six months.
The point is, that when these hormonal boys are going through these very confusing times in their lives, they are just having a hard time understanding their own bodies, what they are thinking, what they are feeling, and what is going on in the world around them. Heck, who ISN'T confused by this crazy world these days?!? When I was going through my surgical issues, I was actually damaged by the first surgeon, requiring me to have the second surgery, so I was pretty messed up. I had a therapist, and I mentioned my son's actions to her, and what I thought was going on with him to her, and she agreed that my suspicions were true. She also told me to hang in there with him, and said that I should support him as much as possible, let him know that I love him, try to know when it is necessary to push in for his safety, and just go with my gut as a mother as to when to intervene if necessary.
It is a different situation, I know, since your nephew is not going to be living with your sister, but I hope this example helps. Best of luck to your family!