First, this may have spoiler so read by your own risk Well, this is all about because I suspect that Emiya Shirou and Kotomine Shiro, aka Amakusa Tokisada Shiro, though are obviously not the same, are related in some way. Aside of just the name, they also have a certain phisical similarities and the most important thing is that in the same way that the ubw wear away shirou, making his skin dark an hair white, it seem the same hapened to the priest Suspicious right?
Shirou's True Name is Amakusa Shirou Tokisada (天草 四郎 時貞,Amakusa Shirō Tokisada?), the teenage leader of the Shimabara Rebellion. Born in the Edo period, he was a boy of miracles who could nearly be called a saint. However, how exactly he was discovered; a good half of his lifetime is wrapped in mystery. He, who concentrated on his studies since childhood, began to perform many miracles at the frontier for a time. Having healed wounds and walked on water, he eventually began to be enthusiastically worshiped as son of God by the peasants that believed in a prohibited religion. In reality, he was merely a Spellcaster. He wasn't taught Magecraft but it was something that came to him naturally but very rarely. His use of Magecraft was mostly subconscious, so he had not even realized it himself. It just so happened that he was born with the power to exercise the miracles called Magecraft, and no one around him ever pointed it out, and so he was revered as a boy of miracles. Perhaps Mori Souiken and his peers had suppressed those whispers. ps: well maybe. . . one of his lifetime? i just copied this on the . . . .
Nah, only the look similar, according to his bio, he forcefully reincarnated himself thats why he has white hair. his skin colour due to he spending time in Africa. He already existed as a servant during 3rd Holy War.
Aside that Miyu can be his sister, there is nothing know about the real family of Shirou, so maybe the priest could be his ancestor