what they feel for each other is "lukso ng dugo" in tagalog, i dont know the terms in english, maybe leaping in the blood? they just misundertood it as romantic interest, this kind of story really happen in real life, here in philippines many cases like that happened when they instantly fall to each other and then later discover they were siblings , you will feel lukso ng dugo to someone blood related to you either mother, father or siblings when you see them for the first time