Re: Sisters
show them stories about girls who encountered problems by forming relationships over the internet. Don't confront your sisters anymore, just say you understand their actions, and explain to them your concerns. Then let them continue to use the internet. Like people said, there's only so much you can do, and blocking it off totally will make them search for other means, and that is more problematic because it will be harder for you to know if anything goes wrong, and the greater privacy will give them the freedom to do things that they would normally feel restrained to do.