Basil had been dating his sister, both in his current life and his past life. While an incestuous relationship was not frowned upon in this world, many would still consider it unsavoury. Most of the people who practised it came from families which were obsessed with keeping their bloodlines "clean." Therefore, many people thought those who practised it were freaks.
Although the 'mind your business' culture in this world was strong, it was still advised to avoid incestuous relationships. Regardless of the stretch of one's ability with the help of Magic, one still wouldn't be able to prevent one's descendants from getting genetic defects caused by an incestuous relationship.
Of course, it was possible to treat the ailment once the child was born but it would be absurdly expensive. Therefore, only big and old families which had close to unlimited resources could afford to practise an incestuous relationship.
