Answer:
Abomination is something that is regarded with disgust or hatred.
Explanation:
From the dictionary definition of the word 'abomination' , we can infer that it is an event or happening or practise that is viewed with disgust, anger and hate.
From the excerpt above,
These outcasts, or osu, seeing that the new religion welcomed twins and such abominations, thought that it was possible that they would also be received
Before we replace the word Abomination with horror, we will get the dictionary definition of the word horror.
Horror is an intense feeling of shock, fear and disgust.
If the word 'abomination' is replaced with 'horror' , then, the meaning of the sentence from the excerpt changes totally.
The community regarded the osu with disgust and deep hatred and they were outcasts because of that but as they saw that the new religion openly welcomed twins which was also considered with disgust, the osu thought they would also be received like the twins.
Replacing the word 'abomination' with 'horror' , the meaning would be thus,
The osu were regarded with extreme fear and shock so they were outcasts but as they saw that the new religion welcomed twins which were regarded with fear, shock and disgust, the osu believed such welcome would be extended to them.