Sounds about the same J.....her son is very intelligent as well but extremely annoying. Has a complete melt down if he doesn't get his way and if you tell him the sky is blue would argue with you that it is green. It really is exhausting and the Doc knows it as well and has had the same conversations as you have. He will run up to her and say things like "Give me all of your hugs!" "Give me all your money!". Then he will talk in baby talk and just do really annoying things. I have also seen him be territorial with her and has tried to to be difficult towards me. One day I got up to get some coffee and he moved in next to his mom and wouldn't let me sit back down. Crap like that.
I do think if my kids were older it wouldn't matter to them however since they are all the same age (roughly) he wants to play and interact with them and vice versa. They are just trying to feel each other out. He just doesn't really know how to be social though and is actually probably better off as an only child.
If my daughters were not expressing these negative thoughts and emotions to me it might be a different story or at minimum it may speed things up a bit. I think they are tired of him being around. My youngest told me that they just wanted to hang out with me and spend time together with the 3 of us. My oldest asked me what should they do when he runs off and cries? They have both expressed the desire to not move from our house as well. Also questioning why me and their mom can't work it out and try again. My daughters have also spent a ton of time around him as well since he has spent several hours during the day with me while I am working at home. Again it saves the Doc money and the stress of finding someone to watch now that school is out.
My daughters come first though. I still believe they are going through an adjustment period. All of them need to be more comfortable with each other and it's just not there yet. Maybe it will never be and if not then that could be a sign that maybe it's not meant to be. The kids need to be able to get along with each other and if they don't it would place stress on our relationship.
