They put the fear in Thomas early, when he was young. They told him that his mother, their sister, was murdered by strangers and that it was not safe to leave the house, ever. They told him that the strangers were outside and were looking for him and would do the same to him that they had done to his mother should he ever venture out of the house. Only, his mother is still in the house. She's buried in the basement beneath the concrete where their father installed the new oil furnace.
Thomas's mother was not their sister, but a stranger whose car broke down outside the gate and when they saw she was pregnant, they invited her in with the supplies that were at the front gate when they went down to retrieve them. They invited her to stay and treated her like a sister. Irene, Maureen, and Kathleen even helped deliver her baby, the boy they named Thomas claiming him as their nephew after they suffocated her with a pillow and buried her in the cellar. They had to kill her, their father told them that she was one of ones looking to murder them and that it was all just a trick to get inside the house and kill them as they slept. They believed him, of course. Why wouldn't they? They were just as crazy as was he.
They named him Thomas after their father, the only other man the three sisters had ever known intimately. He was a horrible man. He had incestuous sex with all three of his daughters. Yet, they did not know any better, as this is the only life that they have ever known, too. What a world. What a way of life for them, so tragically unfair for three beautiful women to be so lost and out of touch with reality.
Their father lost his mind when his wife died delivering the last of her babies. The doctor was summoned along with the midwife, but they were killed along the way when a bolt of lightning spooked their horse and their carriage was thrown from the mountain road. Their bodies were never recovered. Their father, Thomas, was left to his own devices to deliver his babies, triplets.
Nearly out of his mind then with grief for his dead wife, the love of his life, and with worry for their babies, he cared for his three young daughters, as best as he could. Only, the more they matured, the more he slipped away into insanity and into a world of his own. When they grew into beautiful women, their charms were too difficult for a crazy, lonely, old man to resist. He bought them dresses, hundreds upon hundreds of dresses and had them fashion the dresses for him, changing and unchanging as he leered at their young, naked 18-year-old bodies.