You will start creating the segment for parent communication under the people section as you would for a student segment. The first step will be to select the "Add Filter" and on the type dropdown that defaults to all, select "Relationships".
This will open up a new window where you will do your segmentation.
You will select your relationship type by selecting the dropdown of "Relationship is" and finding the relationship type you want (ie Parent to Child, Sibling to Sibling, etc.). This will add a new line asking you who are you trying to pull from this relationship? (ie, you are filtering based off the students data but you want to result in the parent profile. You would keep the "Parent" button blue.)
After you have this set up, you just start adding your filters based on how you want to segment. This is just like segmenting for students, the only difference is it will result in any profile that is related to the student instead of resulting in the student profile.