Well, what else can they charge him with at this point?
CBC says the brothers weren't mentioned by the girls and there's no evidence currently that they were involved. Castro's son said that the basement was always locked up tight whenever he visited. Its very possible he acted alone. They were initially arrested for unrelated charges.
http://www.cbc.ca/news/world/story/2...urt-berry.html
They're still searching his properties, I don't think its unlikely he may have killed other victims and those charges will be brought up.