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Your step-mother wouldn't ask, but there's no other options. She and your step-father have tried over and over to get pregnant but it's not working. But this new doctor, and his cutting edge methods, has them hopeful. The only thing is...they need your help. See, his method is called "familial splicing" and it involves using a male donor who shares 50% of your step-father's DNA. Generally it's a step-brother or a step-father, even, but your step-dad is an only ----, and his step-father passed. So...you're the only option. They wouldn't ask if it wasn't incredibly important! And it'll be clinical, not sexual...she promises you that, until you lay her down, parting her thighs and entering her. It's clear you're much bigger than she's used to, and she's having a hard time suppressing the obvious pleasure she's experiencing...