Questions - Please Help

I'm confused - I wasn't able to pay full attention to the movie while I was busy doing other things - I understand that Rick & Maddie are part of an incestuous family line but is his old girlfriend related to them also in some way? Why was she chosen? Was Maddie pregnant at some point & self-terminated the pregnancy? Could the girlfriend have possibly been impregnated artificially w/Maddie's embryo before she & Rick had sex?