That sister of him is made of hatred or what? She seem not a human being? #－.－)

Nah, the sister just can't show emotions like normal people do. It's cause of her protection(?) or power(?) forgot what they're called.

The sister actually loves her brother very much and can only show emotion when she's around him.

Is that so? (⊙o⊙ ) then it's my misunderstanding.(￣∇￣")
Where did you read it?ヾ(☆▽☆)
Can you share the link, pleaseヾ(❀╹◡╹)ﾉ~

You can read the light novel here https://www.novelupdates.com/series/i-was-kicked-out-of-the-heros-party-because-i-wasnt-a-true-companion-so-i-decided-to-have-a-slow-life-at-the-frontier/

Thanksヾ(❀╹◡╹)ﾉ~