I think it must be the level restriction you have set.
When u first start playing Diblo II X it automatically ticks a box where it restricts characters who can enter your game.
I think the default level restriction is + / - 5 levels around your own level.
Lets say its set to 5.
U r level 10 and you create a game.
Your brother is level 20 he tries to join your game but cant, as the maximum level it will accept is level 15. With the level limiter set in this case the limit is Levels 5-15.
I would untick the limit box , or change it to like 90.
Hope that helps.
(P.S) The box is in the area where u create new game, u will see options like setting the difficulty (normal , nm, hell) , game name, password and there will be one with the level limiting box.