Another option..just to make things more confusing..might be that he some space issues. This may not be applicable as I adopted Kavin when he was 10 months old and as he became more comfortable with me he became less "friendly" to others. This is most apparent with children and my dog trainer said that he has issues with strangers being near his head. I do a lot of handling of all parts of him, so he lets me do things, but not other people. Anyhow, because the youngest sibling is literally "in his face" it evokes a different reaction. He is establishing boundaries he doesn't need to but he doesn't realize that, maybe. Anyhow, when I learned that it changed my approach to Kavin completely. Also, I had enrolled him a GROUP obedience class so we could work on socialization. He has social issues. It has done him a lot of good. He is still not completely predictable but he has gotten a lot better.