Thanks to all for your comments. After considerable thought and testing I have decided to create two custom facts.
1. Relationship to father.
2. Relationship to mother.
This approach will for the most part mirror what I had in my previous genealogy program (Legacy 7.5).
I simply do not believe the birth fact alone is sufficient. As I stated earlier an individual can have a Relationship to multiple fathers or mothers. I do not feel that the place to source adoptions, custody awards, etc is a birth fact.
Sometimes it is necessary to for go simplicity in order to gain clarity. Again thanks for taking the time to comment.