Why is my sister acting like this?

Ever since my sister found a boyfriend shes been mistreating the whole family. Shes been lying to my mom, treating me nasty, always yelling at my little sister and being disrespectful to my dad. I do not know why. Is there something wrong with the boy shes seeing or is it just her attitude. Shes changed significantly and it makes me feel bad. Please answer this cause i need to know why. Before she got one we used to be like best buds, now it all downhill. :(