B.H.
It sounds like your right on! I would try and tell your sister as nicely as possible that your only telling her this because you care. I would start out with a compliment "I think your a great mother"...... then say you think the boys behavior is a cry for help or whatever because he misses mommy. Say how wonderful the child is in your care or whoever elses and that she should atleast try spending more time with the child and see if that helps before turning this into a misdiagnosed medical problem. Just wondering also how he is in school? You could almost say she's neglecting her children emotionally on that.
BTW I've been meeting, seeing , or hearing about alot of parents doing the same thing lately or partying all the time. And everyone of these types I've noticed has horribly behaved children. I feel so bad for the kids in these situations. Once your a parent your life should change and adjust and partying every weekend isn't acceptable. Don't get me wrong I love to have a girls night out or go the bar once in awhile but I really enjoy taking my little girl to do something fun too. I'm not a parent because I have to be I'm a parent because I want to be and there is a HUGE difference in that statement.