Python: Not able to import module from sibling directory

Question

I am using Python 3.4

I have cloned a git repo which has structure like this:

Parent_dir/
      .
      .#random things
      . 
      Sub_dir/

              Sub_dir1/
                    __init__.py
                    file1.py

              Sub_dir2
                    __init__.py
                    file2.py

              Sub_dir3
                    file3.py

              __init__.py

Content of file2.py

from Sub_dir.Sub_dir1.file1 import attibute1 #OK
from Sub_dir.Sub_dir3.file3 import attribute1 #gives an error that there is no module named Sub_dir3

I tried to add __init__.py file to Sub_dir3 but still it gave same error. Then I tried to experiment with this so I created my own project with similar structure but in that case everything works fine.

Is there something obvious that I could be overlooking in the original package that I am working with?

My IDE is PyCharm.

I looked through these1,2, questions, but couldn't find the answer.