okay so you know the shack by the villa?

That little locked shack becomes the home of Nina's husband when she dies.

Don listen to the person above me. Galens home is built on that hill with the tree across the stream in year 2. The little shack is probably useless but I dunno

The shack is useless by my reckoning. It's probably something like the unopenable shack on your farm. Maybe it was originally meant to be Sebastian's quarters and they never took it out. It's frustrating but sadly there is no good answer.

ACTUALLY there IS a good answer. Unfortunately, it IS useless. BUT!!!!!!!!!! If you are a Harvest Moon Freak like me ;) well then you'll know EXACTLY what it's meant to be! I've played all the Harvest Moon games, and there was this interesting thing I found out. In Harvest Moon DS, there are both the Harvest Goddess and a Witch, and the Witch casts a spell on the Goddess, and the Goddess disappears and you must unlock at least 60 harvest sprites to get her back. Meanwhile, the Wicked Witch actually LIVES in the shack next to the Villa! It's full of bookshelves, so it looks sorta like a library, but it's extremely messy and there's a giant pot in the middle that the Witch uses to conjure up some potions. But other than that, the shack is COMPLETELY useless. I hope this helped..
<3,
Nami :3

The shack is where you find a letter from your dad... It can only be opened the last day of your life Winter Day 10 Chapter Six trust me I have opened it...