When the insane dog terrorized the girls, Sarah, the youngest sibling, volunteers to venture out for help against the muted objections of the older girls.

This seems more like a scene from a story than an actual story. How about this:

"After being trapped by a rabid dog, a young girl plays a deadly game of cat and mouse with the creature in order to bring help for her critically wounded sister."

That's still a little bit fuzzy, but having one of her sisters injured or possibly dying, you give a more compelling reason for the girl venturing forth in-spite the danger.

Hope that helped, good luck with this!