The remote install edition can be found here:
Silent Logger Remote Install Edition is a remotely monitoring software
How technically inclined is your sis? If she is very technically inclined, there is a good chance she will know it's there, but for the average user, probably not. What it does is logs every keystroke entered into the computer, your mom can then go in and see what she has been doing.
Be careful though, software of this nature can be really shady. Google it and look around to see what you find.
Oh, and BTW, you don't wanna know the opinions of the people around here regarding keyloggers

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Originally Posted by Silent Logger Plus Description
Silent Logger Plus is ideal if you need to discreetly monitor your college student's computer or the activities of an employee.
(emphasis mine)
At what point does one leave the prying eyes of one's parents?