What do I need to do in order to adopt sofie?

Solution 1:

In Skyrim (with Hearthstone DLC) you can only adopt a total of two children. Quoted here by TheNeeoo:

The cap is two, regardless of which property you use.

Also, for others, if you have trouble adopting children, make sure you follow the requirements shown here:

In a homestead, a child's bed can be fitted in the Bedrooms or a single bed fitted in the Main Hall. When in the Main Hall the dresser in between the two single beds counts as a chest for both possible children. Otherwise, if there are two child beds, there must be two child chests. If there are two beds but one chest, adoption will still not be allowed.

There may be other certain requirements not mentioned above that depend on homes and upgrades.