You do not need to pay taxes because your name is on the title already.
If your name was added prior to your father's passing, then your basis is the same as 1/2 of his basis. You will add to your new basis whatever you paid your sister.
Unfortunately, because you were on the title prior to his passing, you cannot get a step-up basis to the FMV on the date of death that would have been available had you inherited the home after death.
For example:
If your Dad's basis was $100,000. Your basis as a 50% owner is $50,000. If you paid your sister $75,000, then your new basis is $125,000. When you figure your Dad's basis make sure you take what he paid for it and add the costs of any improvements he made while he owned it to get his basis. Basis is not always the sales price if improvements were added.
