That scenario is possible, but obviously weird. Your mother's mother could be be as young as 26 - 34. Let's say she's 36, and your mother was 18 when she had you. Your dad could be any range of ages, but let's say 30. If you mother died, but you Grandmother were still alive, he could conceivably marry her and have a child with her. You never specified the ages , nor the degree of relation.
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