Find relation by number of members

In an own map I have a special relation type which only makes sense with at least two members, one in the role left and one in the role right. So to find critical relations it would be a good first approach to filter all relations with only one member. Is this possible, like tags:5-10?

A little bit more advanced was my approach was to select all relations, and then remove the ones with this kind of member configuration:

parent role:left child role:right parent, but this expression seems to be odd (at least after the parent role:left part.

Basic idea:

• select all members in the role right
• select their parents (=relations)
• from those relations, select all children role=left
• again, select their parents.

Can someone there give me a hint?