Stamp Duty Query - Buying Half Share Of House

[BobbyFowler]

Myself & my sister bought our first house 5 years ago. It cost 255K. It's now worth 430K and 180K remains on our mortgage. I'm going to buy my sisters share of the house. So I'll have to remortgage and give her 125K. I'll therefore be looking for a mortgage of 305K. I'm not sure of the Stamp Duty Implications. I do know that if there is Stamp Duty applied, that it's halved as I'm buying the share from a member of my own family. Is this viewed as me buying a new property or is the fact that it's still the first time buyers home taken into account. So my question is - Do I have to pay Stamp Duty & if so, how much is it?