My advice is to ask your sister to apply for a visitors visa under the relative category, your sister would need to demonstrate that she can sustain her visit for up to 6 months without recourse to public funds.
You can search the UK Visa website for information about the categories in which an au pair can be allowed entry to the UK, if you wish your sister to be a ya ya, I know of one successful visa recently, where a sister has been allowed entry to UK for 6 months, thats a good start.
You may find getting your sister to UK to be a Childrens Nanny to prove difficult, but not impossible.
You would need to show that your sister can be accomodated at your home in her own room, plus to show what her salary would be.
I am afraid, I cannot be more help than that, perhaps one or two others on the forum may have gone through this and can shed more light.
