To progress to find your brother, you need to get past the broken bridge in Velhora Waterfall. Upon heading there, you'll find an NPC called Jensi, who is more than happy to help cut down a tree, but with the help of his friend Farven, they would be able to do it before nightfall. Jensi asks you to deliver a letter to Farven as proof you've spoken to him.
To find Farven, you must go to Velhora Waterfalls, but to a different area than where you are now. This other area cannot be gotten to from where Jensi is located. You need to go back to Raven's Road Camp and enter the door to the right of the cooking pot. Go down the ladder and into Velhora Waterfalls.
Upon entering the area, continue down until you see the ladder. From here, go left by climbing on the vines or jumping over. Keep going left, then fall down, and keep going to the left at the seesaw/scales.
Bounce on the blue mushroom in the thorns to get to the other side. Be careful of the insect nest that will continue to spurt out bugs until it's destroyed. Keep going left, then fall down, as a gate is blocking the path to the left. Down below, you'll see another seesaw.
To the right fall, a tiny bit, and the room underneath the seesaw is where Farven is located. Give him the sealed envelope, and you'll be teleported back to Jensi at the broken bridge.
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