This may not be entirely useful in this situation, but if you wish to report your mother anonymously for dangerous driving, you can via
CrimeStoppers.
You can anonymously report her dangerous driving, giving accurate information.
CrimeStoppers will then forward this onto the relevant authorities, mainly your Police.
I don't know the end result of that, but it could be that your mother is questioned about her driving.
Potentially she could recieve a fine/points on her license, but I'm unsure how this would work coming from an anonymous report.
At the very least, I think she could recieve some warning about her driving.
At any rate, the website is useful for reporting crimes anonymously.