Thank you for your reply..
This is relevant to your case
Married couples & civil partnerships
For the purpose of property ownership, married couples and civil partnerships are seen as one unit. This means that if one person already owns a property, any subsequent residential purchase by either person will be seen as an additional property for both individuals.
Married couples living separately, who are separated in circumstances that are likely to be permanent, will now be treated individually and not as one unit.
Stamp Duty does not apply to inhertited property so there is no SDLT to pay. If someone who has inherited a property then goes on to purchase another home this could be counted as an additional property.
If however someone has inherited a small share (50% or less) in a single property within 36 months prior to purchasing a new property, they should not be liable for the higher SDLT rate.
This exemption has been introduced to provide flexibility for purchasers who may find it difficult to dispose of a share in an inherited property.
You say her name was added to the deeds in 2012 (more than 36 months prior to purchasing a new property), this changes the scenario..
Having considered both aforementioned points, it appears you would be liable to additional 3% stamp duty. I am sorry if this is not what you expected.
Please only rate my answer if it is acceptable.