Dog barking viciously?

My 8-month old black lab/blue heeler mix barks at my 14-year old cousin every time she's in the house. She normally does this with strangers, but my cousin has been coming up to my house frequently but she still barks at her viciously. She did this with my grandma the first time but now she does not. When my cousin spends the night, she has to be very quiet when she leaves my room or she'll wake up my dog. I don't know why she does this or how to get her to stop. Any suggestions?