I would think that, as he's had you to himself all day long, he realises when the others come home that all your attention isn't solely focused on him, hence the crying etc.
If it's not that, it could be that he wants them to play with him and obviously they're busy doing their own thing.
My ds is 2 also, and is an only child. However, my nieces (aged 4 and 6) sometimes come round after school and although he's not naughty or crying, he becomes extremely hyper and excited and wants to play with them. Being their ages, they're not interested in "playing with a baby" so I have to try and keep him from jumping all over them. Although a pain to have to do, I just have to be quite strict and tell him in simple terms, you don't do that (ie leap on one) because you'll hurt them and if you do it again you'll have to go in time out. And there have been occasions he has had to go straight into time out for it (after his warning), but he's becomming much better. I haven't had to tell him for a while.
It's a hard situation, as he's in his own home and I feel like I'm punishing his excitement in a way. But at the same time, other children, as do adults don't want another child clinging to them and jumping all over them. So it has to be done.
They're still typical 4 and 6 yr olds and complain that he's following them etc, but as long as he's not physical in any way I'm not going to stop him wandering around his own home and watching what they're doing. If he was purposely doing something to spite them (like your example of standing infront of the tv) I would physically remove him with a warning that if he continues he'll go into time out. If it's not being done on purpose you could gently explain that X is trying to watch the tele and if he is standing there then X can't see and he'll be upset.
In terms of your other ds's bedroom - would a stair gate or something stop him having access to that area? If not, I'd probably following the warning/time out thing.
On top of all this, so that he feels involved at some point with you all as a family, I'd introduce some family games etc that you could all do together. In the summer you could have some outdoor games in the garden (football or something).