Many people arrange to take steps to set up a Lasting Power of Attorney (LPA) so that their relatives can take important decisions on their behalf if they become incapacitated.
There are two types of Lasting Power of Attorney. One which covers health and welfare and another for property and financial affairs.
However, this is not possible once a relative becomes incapable of making such decisions and doesn’t already have a Lasting Power of Attorney in place.
Under these circumstances you would need to apply to the Court of Protection to become a deputy to support your relative but this usually takes 2-3 months to set up.
For more information on becoming a deputy please see: https://www.gov.uk/become-deputy
General information about making a Lasting Power of Attorney can be https://www.gov.uk/lasting-power-attorney-duties