I think the math is fine?

2025 - 1927 = 98 (years since grandma was born)

98 - 34 = 74 (years between grandma's birth and OP's)

74 / 2 = 37 (average age of grandma and OP's parent when OP's line was born)

Note that OP's parent could be her father (assuming the research didn't reveal it was her mother), which means that one generation might not need to avoid menopause age. Grandma could have had OP's father at 27, and then father could have had her with a 27 year old (when he was 47) and it would all work out. Hell, even at the extreme, gma has father at 12, then father has OP at 62 with someone of any age.

Not saying it's right; 12 year olds should be playing anything other than baby factory and housemaid to some old fuck. But the math seems fine to me.