Hi,
I have some more information for you about this quest. There are 4 steps needed to be done.
1. You need to obtain the lock-picking kit from an NPC in Item Shop in Vierna Skyland
2. Use the Lock-picking Kit on the lock outside Meryl's house
3. Talk to Meryl
4. Find Meryl's brother somewhere in Herbst Steppe
5. Talk to Meryl at her house in Vierna Skyland.
At step 4, you can find two men talking near a small pond in the map. It is a small island where there are three bridges leading in. Meryl's brother is the guy on the left.