I am asking for my 55 year old sister who has Cancer ..
is she able to retire due to her illness ?
I guess apply for disability would be the reason
Your sister can apply for Social Security disability benefits (SSDI), but she couldn't claim Social Security retirement benefits prior to age 62. If she qualifies for SSDI, however, it would essentially be the same as drawing her full retirement age (FRA) benefit early. Her SSDI benefit rate would be equal to her primary insurance amount (PIA), which is essentially the same as what she'd receive if she filed for her Social Security retirement benefits at FRA.
The requirements for SSDI eligibility are explained on the following Social Security website: https://www.ssa.gov/benefits/disability/qualify.html.