ヒカリちゃん is a so-called ツンデレcharacter. アニキ is the main character of the story and ヒカリちゃん is noticeably harsher with him than with anyone else. 友達さん is curious as to why and the above is how she responds. 友達さん has a verbal quirk in where he ends his sentences with も. It is not the particle も so please treat the sentence as “なんでアニキにだけちょっと怖い ? “ .
I’m curious about the meaning of ビリビリ in this case. Any thoughts are very much appreciated. My apologies for the trouble.