What is this weird red blotch on my sister's arm?

We just noticed it today.
She’s 9
It doesn’t itch
It doesn’t hurt to touch or push on it
But it looks really weird. It’s red with brighter red spots all over. It’s only on this one part of her arm, and nowhere else on her body.
I just thought of this, though – what does the chicken pox look like? We know a family that just got over them, and neither my sister or I have had the chicken pox before.