While fighting with a group of survivors against deadly man-eating monsters inside the underground laboratory, Rita is trying to find her missing brother. Version 2

Thank you for all the feedbacks ;-) working hard to make it finaly right ;-)

Geno, good advice... although... 'she thinks' and 'may be' look redundant. I suggest either:
"... who she thinks created them." or "... who may be responsible for creating them."
personally I'd go with the first as it seems 'stronger'.

Or for a bit a drama: "... who she blames for creating them." Which might explain the estranged part.

No need for "young", IMO. Not crazy about "traumatized" either. If she's the protag, that quality trait will eventually wear off, and frankly anyone in that situation would be traumatized, so it doesn't really separate them from anyone else.

"Somehow involved in the chaos" sounds to me that even YOU don't know how he's involved. This part is too vague. Do not hesitate giving info to make the story clear.

"When trapped in an underground lab, a resourceful (adroit, intrepid) woman must fight man-eating monsters to find her estranged scientist brother who, she thinks, may be responsible for creating them." (28 words)

With this logline, I've described the woman in a more active, heroic, way, and added "estranged" to ratchet the conflict between her and her. Making him responsible for these monsters makes the reader wonder if he did it on purpose or not.

Hope this helps!

Geno Scala (sharkeatingman), judge