I know you said you did not want any more hardware but… for Less than $100 yiou can get a router/switch and run both puters through it on one ADSL or DSL line.
This gives you both what you want and your Mom can keep a full account or run a ‘bring your own access’ account.
But most importantly, this gives you a hardware firewall to work in conjunction with you sofreware firewall and other safety features.
I personally will not run a ‘hot’ connection to the internet such as cable or ADSL or DSL without a router/switch between me and the net even with only one puter. I have run up to 5 puters on a single DSL line and other than when trying to run a big download such as a movie from some site, the slow down was only noticeable during a big down load from a fast site with a big pipe or if someone was on line gaming, which we do not do. We warned if we were going to do a big - high speed capable download.
For surfing, IM’s, message boards etc., there was not much noticeable slow down with 3-4 active users on at one time. I doubt your Mom is a big user at the same hours you are on line.
And if she chooses to go from the full access $20+ charge to the ‘bring your own access’ charge of $11 or less ( I think ) for AOHel, then you would recoup your investment in a router/switch ( I like the Linksys ones ) in just a short time.
IMO, never run broad band without a router/switch between your puter and the net…