It's more about overall look and feel, which in large part comes from the glyph proportions (the x-height), spacing and weights - https://i.imgur.com/mygqn3H.png

Glyph proportions between Whitney and Nebula are almost identical. As are their weights. Source Sans is substantially heavier and more dense looking.

While individual glyphs may be closer between Nebula and Source Sans, but the overall feel of Nebula is that of Whitney.