It only took half a second after hearing the men on the other side of the now locked door, for the implications to click into place. There was no need whatsoever to try to block this door; those men would not be joining them.  Moreover, there was clearly something very suspicious-- something very important-- upstairs that they were trying to protect from them. Whatever it was though, it was NOT her brother, and that was more important right now.

Wasting no more of her precious time, she bolted back down the stairs as fast as her short hairy legs could safely convey her over such dangerous terrain, before rocking back up on 2 feet, snatching the "shield" she had laid aside, and catching up to her sister with a faint scuff on the stone flooring as she slowed herself to avoid making noise, or being carried out into the lab by inertia. She looked wild-eyed at her sister, who nonverbally plied her with questions that she just silently indicated "not now" to.

She and her sister just waited for the inevitable to happen.