FMV is not relevant if it's more than the donor's basis. You need to find this out as well as when they acquired the property. if FMV is more thn donor's basis your gain is the difference between sales price less selling costs and his basis. His ownership priod is added to yours for determing short vs long-term capirtal gain. since you were the only one on the title when the property wa transferred and then sold, I think the IRS would argue 100% of the gain is yours. Consult a pro if there's a gain and you want to split it with your sister to see if this is possible. I don't think that a loss would be deductible since your holding period was so short. Again you may want to confer with a pro.
your step-father needs to file a gift tax return
you may need to file a gift tax retunr for the proceeds you're gifting to your sister
