We're assuming it was a grown man in the case of Lina, though I agree on average, it is grown men.

But Lina was one of 10 children. Where was she in birth order? How many older brothers did she have?

And that brother might not have understood what he was doing either, or the consequences to Lina. Don't forget, it was a different era, there are a lot of sex taboos in Catholicism (talking about it), and if her condition was genetic it's not inconceivable that one of her brothers was also precocious, etc. It is not unheard of for boys raised in deeply conservative religions with a lot of sexual shame to walk in on his parents, not understand what he was seeing, get yelled at, and then act out what he saw with his sister, having little to no idea what he was doing.

If it wasn't the father, an older brother is the next most likely explanation.

After that, male relatives that don't live in the same household but have access and trust.

That religious festival that was mentioned is a decent explanation too, but much less likely.

And of course, her father could be lying and would have been protected. With 10 kids he was pretty fertile. The family would have needed his income to survive and if he had a high enough sex drive, again with masturbation being considered a sin in Catholicism, and likely lack of access to birth control, Lina may have been seen as safe to relieve that urge so there wouldn't be any more babies.

Did anyone ever ask Lina's sisters of they had been sexually abused and did they understand what that meant at the time? It would be interesting to see the notes and documentation on that investigation, wouldn't it?