If your "cuz" is under the age of 18, her mom has every right to check her messages. If she is doing nothing wrong, then there would be nothing to worry about.

First off, does your cousin change their password and your aunt somehow "guess" it? It may be as simple as changing the password if your cousin hasn't changed it since. However, that is so obvious that either the situation is more complex or I should stop now because nothing I could possibly say from here on out will make sense to you. If the aunt is actually hacking the system (in the sense of actively subverting the security measures on the Myspace servers) then there are legal issues which could potentially lead to auntie's arrest. Unless you either want to get really good at computers yourself, it's best to let the pros handle it, even if it means prosecution. If it's something more local like a keylogger snatching your cousins password, then it depends on whose system it *really* is and how old your cousin is. Most minors do not own their own computers outright as most did not buy it themselves. If it's the aunt's computer (or a shared family one) then there really is nothing that can be done. If your cousin bought and paid for the computer themselves then that's a bit different, and different places have different ideas about what rights parents have to snoop on their teenagers so age and location may be relevant here too.