Really, hiding them wouldn't have been a good idea to begin with. Shane had her ways of finding things out, especially when it came to her little brother - mostly because, really, he was one of the few good things in her life. She had to keep it that way somehow.
She wasted no time in coming out to him. Seeing the bruises was... definitely a cause for concern, but right now? The most important thing was Shay. "Hey. Long time no see?"