Well, telling here you have a no-smoking policy in your house will do you no harm. May keep her away a bit more too.
Your cousin is 13, able to make her own choices.
If the mother really does bother you and your friends, I think you have to set boundaries.
If your aunt thinks she needs to know what people her daughter hangs out with, let her in once every fortnight at the most. And tell her not to smoke inside your house. If she does smoke inside, her scent will linger there so long that you hardly have the idea that she is gone when she is gone.
Think it is time to become a little more firm Butterflies. Your cousin may love that too. Gives her a chance to be somewhere where her mother won't barge in to whine. She is 13, and what you describe, she probably already makes quite a few choices of her own.
"Is she gone yet?"