How do I handle this?

A few months back my sister had a party at our weekend house and though it was her and her friend's party, some of her friends insisted on inviting me. I agreed and spent some time there but after some week few of her friends are now asking for my number when they came home later and spending time with me which i am not at all amused by as they are 6 years younger and also sis's friend. My sis doesn't know about it. Should i tell her or should i meet them up and settle the matter and tell them my view?
How do I handle this?
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