When you die, any money left in your SIPP can be passed on, usually free from inheritance tax. If you have a named beneficiary (or multiple named beneficiaries), this money can be passed to them.
Any withdrawals made will be tax-free if a) you die before the age of 75 and b) the benefit is claimed within 2 years of your death.
If you die after the age of 75, or if the benefit is not claimed within 2 years – any withdrawals made will be subject to income tax (based on the rate paid by the person making the withdrawal).