My niece is moving in I'm nervous in damaging our close relationship what do I expect?

I'm 40 year old single mom and never been married. I have 19 year old son, and my 19 year old niece is moving in. she's a great kid and is like my broke best friend, since I don't have any daughters and a free guest room in my house. We are super close, and I don't want to damage our relationship but we are both a bit high maintenance and she is also absolutely gorgeous. She already has a strained relationship with my brother and his wife, which is why she's moving in... I am worried about having her move in and damaging our relationship. Since she's not my daughter what kind of rules/polices should I have? I have always been tough/strict with my son and his curfew and school etc... This isn't a free for all but my niece also isn't my daughter and I'm not raising her, she is already talking about going out partying with me and picking up guys lol which is cool but I'm worried about her thinking this a free for all!!! I'm old and paranoid and need my sleep and peace and quiet this isn't a frat house lol what can I expect and what kind of rules do i set?

My niece is moving in I'm nervous in damaging our close relationship what do I expect?
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