I assume that by "Found it!" you mean that you found a posting that explains that the bash shell does a reverse DNS lookup using your IP address to ask the DNS server(s) what name is associated with this IP address.
And the DNS server (most likely your home router) last used the IP address you are using with your sister's iPhone, and your sister's iPhone registered the name "sophias-iphone" with the DNS server.
You can either clear the DNS server so it forgets that IP address/name association, configure your router to assign the same IP address with the same computer when you return, or modify your bash shell initialization script to use a customized PS1 prompt variable that will not do the reverse DNS lookup.
If you did not find a previous post about this issue, you can search the forums for "reverse DNS lookup" and I'm sure you will find more than one posting that explains this and way to eliminate the problem going forward.