Do you think they let his sister watch before it goes live. This isn't just the odd 2 min clip when they pop round this is full vlogs. Do you think she knows the risks of her children being on YouTube?
And has she sat down with them and explained what YouTube is and how the world can watch and ask for their consent. Although I'm a firm believer that at that age they cannot consent to YouTube due to their age. So even if they did ask them it's not enough but would be better than nothing