When I asked about her (we thought she was over 18), because male tenant said "we are thinking of having her stay with us for the summer" (violation of lease which says visitors can stay 7 days), the female tenant replied "he misled you, sister visits for help with homework, she spends the night sometimes".  

In fact, go talk to the housing director for all the law schools in your area, and ask them if you can advertise your rentals to their students. 

We were informed by the male tenant last week that the female had moved out.

If it's a condo, check out the financials of the condo association and be absolutely sure they have money and they have a good leadership team in place such as a board of directors or management company.

If you have a law school nearby, go talk to the housing director and ask if you can advertise on their facebook page, or website or at least on their bulletin board.Once I discovered law students as tenants, the angels sang! 

I first advertised on the law school's facebook page via the law school housing director (university was next door), then if no bites from there, I'd list on Craigslist.  

I have a tenant (single female) on the 1st floor who has only been there a few weeks and has sex quite loudly around 2am when I am trying to sleep after putting in a 12-16hr workday.

Unfortunately, when the director of the agency that dealt fairly retired his successor was just as unreasonable as the directors of the other agencies.

If you're not within walking distance, then you need to be cheaper than the options that are closer.To target law student specifically, contact the law school and ask for the housing director. 

And are you renting to females or males or co-ed?