I want to duel my brother. Idk how. help please?

You can't choose to meet up with someone in this game, it isn't about playing with your friends.

If you're within the same level range as each other the best you can do is agree on a spot to throw down a sign and hope you can see it.

You must be within a 10% level range within each other to be able to summon each other or PVP, so theres not much you can do without him getting stronger

Go help him level.

Brotherly bonding is always a good substitute for jolly cooperation.
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An alternative would be to go through the "Battle of Stoicism" (part of the expansion)

If I can recall, two of the brackets are 50-100, and 101-200

so if he's level 50/101 and your 100/151, I don't see why you cant deuce it out there

Try the red stone, i know my mate summoned me to a duel and he was well above my level. Just place the stone where noone will see it and tell the bro where it is.

Another way is to go a quiet area and get your borther to invade you, you can invade anyone above your level. I got invaded by a lvl 30 on my 120 character a few times.