You need to specify fontfamilyoptions: sfdefault in your YAML header.

If you were using LaTeX directly, you would have to write this line:

\usepackage[sfdefault]{merriweather}

The part in square brackets is the package option, which you have to specify to get sans-serif text as the default. This is the case for most sans-serif font packages (look on the LaTeX Font Catalogue).

---
title: bla
fontfamily: merriweather  
fontfamilyoptions: sfdefault
...

# Test

Hello, world!