I went through the same situation there a while ago and it does work out handy enough, if your sisters ex partner only has this home then he will not be required to pay stamp duty on it as he has already bought it he is just buying the second half of it, also it is best to get an independent valuation done once a value has been confirmed your sister should receive half of the equity in the property, she will need to get her own solicitor as you cannot have the same one this would be conflicting interests.
Hope this helps