Why can people on my local network play on my online minecraft server but not online?

I was trying to get my cousin to play on my minecraft but he can't but my brother can its weird

thank you ~thetcat

Added (1). but how do i and is it free?

The

You need to port forward the router and to get your cousin it's address.

Without the port forward, your router will ignore minecraft packets and delete them Port Forward tells your router what to do with unexpected minecraft packets.

David E

Because your brother is on LAN and your cousin is not, it is how networking works! To have your cousin play, you have to port-forward.

EDIT-
It is not easy and I don't recommend doing it but here you go-http://portforward.com/softwareguides/minecraft/portforward-minecraft.htm

Austin