You want spoilers? I'll give you spoilers, the story is deviating a little from the LN, I can tell you what happened in the original LN to give you an idea of what the sisters are capable of, but be warned, it's bad and it's going to get worse!
And for the other spoilers
And lastly
And you're absolutely right about that! Because when they will actually start asking for an explanation from their brother, they WALKED OUT from said explanation, which they asked!
Yes, only volume 4, readers had to power through said volume and its insufferable little sisters characters. But I...
Here comes the worse part of this story, so bad some people dropped the LN at volume 4.
Oh, but they call their brother “onii-chan” or “nii-nii”, so it's cute, we're supposed to forgive them and let it go /sarcasm