Tia and Tamera are smoking cigarettes, and they do not want their parents to find out. They feel that their parents poke into their privacy too much, and they are fed up with it. Still, sometimes, parents need to get a wee bit nosy when it comes to serious matters like this particular one. Smoking is absolutely dangerous and can kill you if you do it too much.
I do agree that everyone needs to respect other people's privacy, but when it comes to life and death situations, then you have to get a little nosy. It's to protect the ones you love from killing themselves and making themselves sick. That's what the twins were doing, hurting themeselves.