A person is entitled to Carers Allowance if they are looking after someone who gets a qualifying disability benefit.
This means the person you are looking after has to be getting:

the middle or the higher rate of the care component of Disability Living Allowance (DLA)
either rate of the daily living component of Personal Independence Payment (PIP)
either rate of Attendance Allowance or Constant Attendance Allowance of the normal maximum rate paid with the Industrial Injuries or War Pensions schemes
Armed Forces Independence Payment (AFIP)

Not every carer can get this benefit. You may be eligible for Carer's Allowance if you meet all the following conditions:

you look after someone who gets a qualifying disability benefit
you look after that person for at least 35 hours a week
you are aged 16 or over
you are not in full-time education
you don’t earn over £120 a week (after deductions)
you satisfy UK presence and residence conditions

Note: If you meet the above conditions but already receive certain benefits (including a State Pension), then the amount of Carer’s Allowance you may be entitled to could be affected.

(taken from Carers UK website)