You are mixing two Python interpreters. For instance, your virtual environment uses a different Python interpreter than the one used to build the dependencies. See the Homebrew recommendations.
To ensure all dependencies and the virtual environment use the Python interpreter shipped with the OS, start from a clean slate (see below) and make sure which python returns /usr/bin/python. If Homebrew Python is installed, you will have to brew unlink python.
The document Homebrew-and-Python explains how Homebrew works with Python.
Check the general Homebrew troubleshooting.
Inspect the output of brew doctor. Not everything reported there is a problem but there might be conflicts, especially if you also have packages installed with MacPorts or Fink.
All Dolfin dependencies can be uninstalled with
brew uninstall $(brew deps --recursive dolfin)
If you encounter a build error (most likely with dolfin), rebuild with brew install -vd dolfin. When the build error occurs, you will be presented with an option to drop to a shell. There, you will be able to look around and inspect the source tree, the partial build, the CMake error logs, etc. Type exit to return to your normal shell.
If nothing gives, feel free to file an issue. If there was a build error, please post the output of brew gist-logs <formula> where <formula> is the name of the formula that failed to build (presumably dolfin).