What is the best way to keep your sister in the house?

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my sister LOVES to go outdoors, she has her own set of friends, most of them are her age, and some are a little older (their ages range from 14-16 years old).. they've been friends ever since they were still toddlers and i completely understand her if she prefer to stay with them, but since they are already teenagers, i don't think it is still best for her to go out with them, i mean TWO of her friends were already reported pregnant, one is 15, the other is 14.. i am so shocked that i forbid her to see her friends for i am afraid she might also get pregnant, i am not saying that i don't trust her, it's just that i don't trust her boyfriend.. she still doesn't tell us that she has a boyfriend, probably she is embarrassed for she is somewhat boyish, but i know she has a boyfriend cause i accidentaly read one of her letters.. well, i was proud at first, but now, i really am not sure.. she's keeping it a secret, i never dared to ask her.. also she keeps seeing whoever this guy is without us knowing, so how am i suppose to know nothing has happened between them? she is still 15 and I really don't like her to have the same fate as her two friends, my father already bought a Personal Computer so that she will stay at home, but she still manages to get out of the house.. my parents already know she has a boyfriend, but they are keeping silent, we are all waiting for her to tell us.. anyways, i'm really worried about her, could you please help me keep her inside the house? i am sorry this is long... thank you for taking the time to read this, i really need advice, fast.. =)