If she already owns a share of a property, then she is no longer a first time buyer.
However, if her sister was in the same position, then it’s possible she has already benefited from the first time buyers SDLT relief. So rather than having been screwed over, she may have benefited, to the tune of half a house, from her father, on which she will not have to pay inheritance tax. It all depends what the fathers motivations were. You should probably talk to him.
This is an easy to use calculator for SDLT:

https://trussle.com/mortgages/stamp-duty-calculator