Retrospectively claiming carers NI tax credits?

[T5GRF]

My mother is 85 and has a state pension which is roughly 60% of the maximum. She only worked for a few years before having children but did make some voluntary NI payments in the 1990’s both regular and one off to boost the pension.
She cared for her father and brother both of whom had chronic health conditions for many years but despite advising her to become a carer and claim carers allowance which would have given her NI credits for the period she refused. Is there any way for her to now ask for credits for the period she cared for her family given she has had the state pension for many years?