Most operating systems have accessibility options to help with that. Consider helping your mother to use those, they exist for a reason. Punishing everyone just because a few folks need help is not the answer.
But flip the logic. Your inaccessible site, due to tiny unreadable fonts, is punishing everyone whose eyes aren't as good as yours, which is a lot of people.
Or just stick to the basic logic: you can increase tiny fonts, where as on most sites now I can't shrink them. So it is punishing.
Your mother is inconvenienced, we are punished. She only has to adjust one setting, where as I get no option.
And nobody is talking about tiny unreadable fonts, we're talking normal verses gigantic. If she can't read the normal OS font size, shes the problem and not the rest of us.