My younger sister likes my boyfriend help?

So my sister which a year younger than me likes my boyfriend , our relationship is secret so he comes my house as a friend. we have known each other for 2 years n are in a relationship from 6 months. My sister always dresses up , use makeup nd other things when he is around. she touches his hair , cheeks and says all variety of things , tries to seduce him and impress him. my sister chats with him whole day , she even proposed him many times as a joke , but i know its not a joke. For Our secret relationship my boyfriend also chats with him n be friendly so she doesn't gets a clue. I seriously hate my sis for these things when i tell her her to be in limits she says " why? who are u to tell me? he likes me and the things i say or do then why are u having problem?". My boyfriend always make me feel secure, he thinks my sister is young so not to worry and just chill , he says he can handle her. help me what to do?
My younger sister likes my boyfriend help?
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