Slapping her surely won't do... As you see her mother is backing
her up, by doing that, you will only make your position
unfavourable.
Help her most likely is impossible since your sister sounds like
those stubborn type, if so, obviously she would not listen to you,
let alone talking about her problems, only your parents can help
her. Do you parents know about your sister situation in school, if
not talk with your parents about it. Might encounter some
difficulties if the 'prositute situtation' happen just
recently.
If not, for me, I would pick a few actions (my brother last time
also like that):
1) Hide/destroy/wadever things you or your family consider bad or
give to a friend you trust to take care first. Let your sister poke
as much as she like. As time goes and she find nothing to complain,
she will eventually back off. Be careful though, she might change
target to something else.
Note: To me, I think give a friend is a bad idea, cause normally I
don't easily trust others. If not careful, might make your life
outside miserable. 
2) Since she know your sg forum ID or check your computer, then
make another sg forum account don't let her know your ID. Clear all
your internet browsing history, so she can't check what website u
visited or u do on the net.
3) Ignore her at all cost/if possible. Don't talk to her
unnecessary, if really a must, make it short. Like that she will
sense that you are really angry with her and she might refrain from
interfering with your business.
4) Endure, never quarrel/fight with her. Nothing good will come out
of it since your mother is backing her up. The more you fight with
her and got scold by your mother, the more happy she is and
naturally she will continue search your stuffs to make her happy.
Ever heard before this quotation? "I learned my lesson never to
fight with pigs; I get dirty, besides the pigs like it." 
Yup, tolerate even if she tell your mother about you and
stuff, just tell yourself "Ok, I lose this round cause i never
cover up my 'secrets' properly, there will not be a next time",
treat it like a game of hide and seek. You hide stuffs, she
seek.
I used this tactics when the times my brother act like your sister,
yea my mother also use to back my brother up. I also learned
something when last time my mother used to scold me and we would
quarrel about it.
The solution is keep quiet, let her finish what she say, don't
interrupt, you interrupt she say you talk back and scold even
longer. Surprising, it works. If I defend myself from my mum
scolding it, the scolding can last to maximum up to 1hrs+ or worst
up to the next day 
, if never say anything it, it only last less than 20
mins. Act good boy during this period of
time, at the back you can be as naughty as you want, just cover it
properly.
Main idea is cover your tracks properly. Keep your mouth close,
don't talk unnecessary. Be rational/cool at all times. All the
problems will fly away 
, though I sounds like trying to help people become a
naughty boy and won't get caught. 
All these my past experiences and the most appopriate solutions
that i can think of, may or may not work depends on individual, but
worth a try. 
