Care allowance

While there is no upper age limit for claiming Carer's Allowance, you cannot receive the full amount of both Carer's Allowance and your State Pension at the same time. This is because Carer's Allowance and the State Pension are both classed as 'overlapping benefits'.

This is from Google. But maybe advice needs to be taken.

Is your step father receiving Attendance Allowance? Carer’s Allowance can only be claimed for caring for 35+ hours a week, for someone in receipt of AA or another disability benefit. There are no age limits, but it is an overlapping benefit, so someone claiming another benefit will probably not get any money, however it is still worth claiming, as an underlying entitlement can mean receiving a higher rate of another benefit including Pension Credit.

Do you have a reason for suspecting your sister is claiming the allowance?

Your mother can claim Attendance Allowance for him.

There is no point claiming Carer's Allowance if you already receive the State Pension as you would lose the equivalent amount from the Pension.

It is intended to replace lost earnings from paid employment.

But, as Cabbie said, it is worth putting in the claim as it can open the door to other benefits.

Two people cannot claim CA for the same person. Who puts in the most hours?

Ifs just that my sister told my mum she couldn’t claim for it as it had said my mum was to old.

she may have been referring to the rule as stated above, that if receiving state pension, a person will not be paid carer's allowance.
but as stated above, it is worth claiming anyway.
this presumes that your father is in receipt of attendance allowance. if not, get on to that.

Caren15 - is your step-father claiming Attendance Allowance? I think he would qualify:

www.gov.uk/government/publications/attendance-allowance-claim-form