Just tell your Mother (and I am one) that you are an adult and that what you do on the net is private, just like what she does and your brother does and your father does and your sister does and what the cat whats to do.
As long as you are over the age of 18, you do have the right to privacy. Either that or save up your nickels and dimes and get your own computer and then make sure its protected. When all the my kids used my computer, each one of us had our own password and no one thought different of it. Until one child decided he needed to be on the computer at all times and then to log on you needed a password. The rest of the kids came down hard on the one.