The airline company does not allow my mother in a wheelchair to sit next to me on the plane

The airline company does not allow my mother, who is my main carer and uses a wheelchair within the airport … to sit next to me on the plane. I need her to seat next to me as I have difficulty and need her assistance in eating and getting to the toilet.

…they now allow their passengers to buy their seat. According to their customer services, this option to buy the seats you prefer is not allowed for people with mobility difficulties and that they have designated seats in the aircraft with mobility difficulties and who use a wheelchair. These seats are by the window of the aircraft. This, of course, leaves us the only choice to book a seat 8 hours before the boarding and the risk is that we may end up in different seats and separated, which of course prevents me from having my carer next to me for my main needs.