Why does my dog ignore my sister?

So we have a family pet - a maltese x shih tzu - and she loves my whole family (especially me). Whenever I leave the room she follows me, and sits on my lap whenever she can. Right now she is sitting at my feet! BUT she completely ignores my sister! Laura calls and she wont come, she holds her but our dog growls. Why does she love me so much but dislikes my sister?!