You may be familiar with these links on Conditional Value Lists:
Forum Tutorial: Custom Value List?
Knowledgebase article: http://help.filemaker.com/app/answers/detail/a_id/5833/kw/conditional%20value%20list
Hierarchical Conditional Value lists: Conditional Value List Question
Take a look at "Option 1" in the Tutorial. It illustrates a conditional value list that does not use a relationship.
If you prefer to use a relationship, you can define a calculation field such as:
constYes: "Yes" // a calculation field that returns the text "yes" in every record.
You can use it as the match field that matches to your existing field of "yes" or "no" values in a relationship based conditional value list.
Feel free to post follow up questions here if those links aren't enough to get your value lists working correctly.
Thank you so much Phil - this helped me get my head round it in a matter of minutes!
All the best.