Advise regarding my sister please?

Multiple copies of the death certificate are needed because each bank will want one, some companies will want one, depending on what companies were involved and how many accounts he had....we had ten I think and have a couple left.
Unless your sister is an executor, she's not entitled to see the will before probate has been granted, and the accounts have been finalised.
As above, if her father was in recipt of benefits of some kind, the DWP will have to be notified. they then check if there were any overpayments that have to be clawed back. This can take a long time [ a family friends wifes estate wasn't finalised for two and a half years due to this].
You say there is a will, so, he may well have left nothing to your sister, you dont know.
He may have had debts, so the state would become insolvent, in which case, it's best not to touch that at all.

His pensions/life assurance [if he had any] and if they were put in trust, will go to whoever is named on the policy or named in the will.
His bank accounts, unless they were joint [ in which case the monies now belong to whoever was named on the joint account] will be accessible to pay for the funeral. if they had any money in them, and then any remaiing will go debts first, then any beneficiary.


All this can take months, potentially years, to sort out, so your suggestion of shennaginans is jumping the gun a bit.