ElaineB:
This would be difficult as Insurers could believe that you were aware of the risk when you booked the holiday. You would need to phone Insurers for quotes to cover the three of you as a group. The details would need to be referred to underwriters, who would look at the risk to decide on whether they could cover and if so the endorsement terms/premium applicable.
To decide on this the Insurers would need to ask quite a few questions about your mums medical condition, to determine how likley the trip would be curtailed, as well as medical costs etc.
If you look at the post office Travel Insurance site, there is a freephone number. Give them a call to run through the quote basics. They may need further medical details, so you may have to speak to your mum and then give them a call back.
Huckster