They'd never been sick, of course they hadn't they were the Daughters of Thanos and they were far above some common sickness that might make a lesser being unwell. They'd been designed as weapons and weapons had no weakness or something like that.
Gamora didn't want Nebula to feel unwell also but it reassured her that whatever it was was effecting them both. When she sat Gamora touched Nebula's forehead with her hand, finding that it like her own was warmer than usual. "We both have a fever," she said with a sigh.
"I don't want to hydrate, I want to sit here forever," Gamora complained as she leaned her head down onto her sister's shoulder and closed her eyes. "They did this to us, we should kill them all for that," she said and it lacked any of her usual conviction.