It's hard to say for certain. Sometimes stories like 'La Esclava Blanca' combine real-life elements with fictional creations to make an engaging narrative. You'd have to do some research to know for sure.
The significance could be that it represents a unique sad story in an English - related context. Maybe it's a story that stands out among others in terms of its sadness.
The phrase 'la triste historia in english the only story' is quite puzzling. 'La triste historia' is Spanish for 'the sad story'. The 'in english the only story' part seems out of place or perhaps it was an attempt to say that there is a sad story which is the only story when translated into English. It could be that there is a collection of stories in Spanish and this one sad story is the only one being considered for an English version or something along those lines. Without more information, it's difficult to be more precise.
Since I don't know the exact story, it could potentially be about a figure who met a watery end and was considered beautiful in a way that defies normal expectations. It could be a story about love and loss, where the person who drowned was remembered in a special way that made them seem the most beautiful. It might also be a story with a moral or lesson, perhaps about the transience of life and the power of memory.
The main character is probably Father Amaro. There could also be characters such as the people in the community he serves. Maybe there are other religious figures or local townspeople who interact with him and play important roles in the development of the plot related to the crime in the novel.