Why is が (instead of に/を) used with せる here?

Question

こわがりの恵美がだんだん顔をこわばらせて、しまいには泣きだしてしまう

恵美's brother read a horror story to her and she started crying. The above sentence explains what happens to 恵美 when her brother reads her such a story. I am having a very hard time understanding why there's a が instead of a に/を here. I found the following example:

あいつらが(私を)怒らせたんだ。

but this construct doesn't make sense in my example because it's not the brother's face that becomes stiff.

Answer 1

It's not about the interaction between 恵美 and her brother. It's causative because 恵美 causes her own face to stiffen. She is indeed the subject, the "doer". If you remove the causative auxiliary, you get 顔がこわばる (face stiffens).