If your mum has decent wired broadband, a pico-cell might be a solution, but network operators do love placing hurdles in the way (for example, EE insist that you get your mobile handset from them with all their bloatware, and even then only a Galaxy S or iPhone).
It would be nice if Ofcom would get off their saggy grey arses and specify and approve a suitable picocell or femtocell that phone users could buy off the shelf and plug in to extend a wireless signal indoors, without needing this to connect via a broadband connection, and without needing network operator permission to do so.
Yeah the broadband isn't great and the WiFi needs lots of repeaters but I did look into extenders and Pico/Femtocells. The problem is she uses EE but the rest of the family are on Vodafone/O2/Three. As we're all frequent visitors (will be there for 9 days over Christmas) we'd need to cover every network. I did see a Celfi extender set in the window of my local secondhand/pawnbroking showroom for I think O2. It was a bargain at under £50 because they obviously didn't know what it was. Sadly by the time I could get there on the next day they were open it had been sold.