Thanks for your question - I am Sam and I am one of the UK tax experts here on Just Answer.
No, you do not need to declare this inheritance to HMRC and no, there is no tax liability arising on this.
The only tax issues that would ever need considering are the first on your late sisters estate - which the executors would address along with the distrbution of your late sisters estate - (so this does not have any bearing on you as this is a cash distribution) and the other which may affect you - is any money you make from investing this lump sum - such as dividens from share purcahses or interest paid on a bank or building socity account.
But both of these will have tax charged at source (before you are paid any money out) so you only have any additional tax liability of your annual income is in escess of £41865 a year - then you would need to advsie HMRC of this arned income from this saved capital sum.
Let me know if you wish me to expand on any of the information I have provided.