Use a payroll company, NannyPaye and PayeforNannies are two that I have some experience with and so I can suggest those. Give them a call (during office hours) and see how quick they answer the phone - often a useful indicator of how easily you will be able to get hold of someone to deal with any situation that crops up.
You must pay at least minimum wage, which I suspect you would be doing anyway.
You must have a contract of employment - the payroll companies can help you with that if required.
You pay HMRC every 3 months at present, so you need to get into the habit of putting the money aside in an account and not spending it.
Agree a Gross salary (that is, the pay before tax deductions). That way if your sister has any strange financial situation now or in the future, you don't become the one paying things like Under Payments, Student Loan. Your sister in turn gets the benefit of any rise in personal tax allowance.
You will have Employers NI to pay... factor it into your cost calculations.
A PAYE Calculator will give you a fairly good idea of what the annual amounts are likely to be for all taxes involved. Be aware that tax rates change, April typically, so your costs will change at that point of the year.
Pension schemes start to kick in... as a new employer registering now, that won't involve you until October 2017 - See here. So no panic at the moment. Your payroll provider will be able to deal with the pension figures, and depending on your chosen pension scheme they may be able to do it all for you.