So in other words, you gave Siteground your Aunt's current IP address and they tried to do a reverse trace from the server. I honestly don't know what that proves given that you (your aunt, that is) have done tracerts to the siteground server successfully.
I would suggest two things. First, create another account on her XP computer and try logging in with that account to see if it changes things. Hopefully, this account will not have any default programs such as Charter's security suite running. But even if they are, it's worth experimenting to see if a simple account change might help the situation. Maybe something's amiss in her present account.
Here's how:
http://support.microsoft.com/kb/279783
Note: this will prevent the pc from booting up to her account automatically. That can be easily fixed, if necessary, after the test is complete.
Open up Internet Explorer from this new account and try to go to your website.
Ok, second thing (after the test above). Someone needs to speak with upper level Charter support --insist on elevating to Tier 2 or 3 or whatever the levels are at Charter and threaten cancellation if they don't elevate the support issue. I would ask them first to forcibly switch her IP address from its current to a new IP, not just a simple renewal of the dhcp lease. See if she can get an actual new IP address. Try accessing the site again after that. She can test her current address (not the internal address from the router, but the external routable address) by typing in
http://whatismyip.com. Get that address before insisting on an IP change, and verify the change by going to the whatismyip.com site after Charter says a new IP has been issued.