Inheritance Tax - personal allowance (parents)

My mother died last year and I have just finished dealing with her estate (my father was the sole beneficiary so there was no IHT). Do I need to do anything now to pass her allowance onto him or does that happen when he dies? In case it's relevant, the beneficiaries will be my sister and me. Any advice much appreciated, thank you.

Hi,
Please refer to the following guidance:
How Inheritance Tax works: thresholds, rules and allowances
Thankyou.

I did refer to that guidance before posting here but couldn't find the answer to my specific question...