Losing your family and village would understandably affect anyone. I’m guessing over time she learned to forget (or at least to not let her mind dwell) in that tragedy.
And to suddenly have someone say “oops, that was my fault” would likely bring back how painful that experience was for her...
(Reposting my previous comment)
Chapter 1 - It says that she is MC’s stepsister
https://mangadex.org/chapter/be4f12bb-b4a3-4b91-841f-f4258ef08432/23
Chapter 12 - It says that her father (a knight) died and Claus’s father was a close friend to him so he adopted her...
Same. Hoping she decides to join Claus.
I don’t see any reason for her to stay in Gaine’s group. Since her thing was to find strong people.
She already saw Claus as strong even before the dragon fight. Now after the fight…
(I imagine next chapter Claus is going to wake up with Charlotte...
Chapter 1 - It says that she is MC’s stepsister
https://mangadex.org/chapter/be4f12bb-b4a3-4b91-841f-f4258ef08432/23
Chapter 12 - It says that her father (a knight) died and Claus’s father was a close friend to him so he adopted her...
That said other adventurers wouldn’t have dungeon teleportation so they would spent a lot of time defeating a lot of skeletons, and thus might gain more levels than MC did.
(Not sure if enough to deal with the death beasts though)