The original poster has legitimate medical needs that warrant separate accommodations. It's unreasonable for the sister-in-law to expect them to jeopardize their health for the sake of group unity. The sister-in-law booked the initial accommodation, and the poster reasonably assumed she intended to stay there. It's selfish of the sister-in-law to prioritize her special time over the poster's well-being.
The original poster is conveniently using medical needs as an excuse to avoid family bonding. Booking a separate apartment without consulting the sister-in-law was a calculated move to prioritize personal comfort over family harmony. The sister-in-law's desire for special time with her daughter is understandable, and the poster is being inconsiderate by disrupting those plans. The poster is exaggerating their medical needs to manipulate the situation.
The sister-in-law's actions of booking an undersized accommodation is the primary source of the conflict. However, the original poster could have communicated more openly about their medical needs and intentions to book a separate apartment, rather than making assumptions. The sister-in-law is being unreasonable in expecting the poster to sacrifice their health for the sake of her plans, but the poster could have handled the situation with more tact and consideration.