From your description of the problems that sounds very much like an antenna problem and nothing to do with networking. Networking certainly would not cause the TiVo to suddenly stop recording programs mid way through, however a loss of antenna signal certainly will.
Maybe while installing the wireless networking you have by accident knocked the antenna cable, or just by coincidence the antenna cable has become faulty? Check that out and if possible while your Sister's TiVo is not doing any recordings of her programs swap the antenna cable and see if that helps. If not see if you can just quickly plug your TiVo in her room and see if the same problem happens.
As I said, this definitely sounds more like an antenna cable issue than something caused by altering a networking component.
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