purchasing home of deceased parent

here is my problem, my father and stepmother recently passed away, there are 3 natural children of my dads and 4 children from my step mother. my sister and 1 step sister are co trustees of the trust my father set up. my sister wants me to purchase the home that my dad lived in when he died, there is still $51k remaining on the loan, the house was recently appraised for $178k, if it goes on the market it will not sell for this price. she states that if I can get a mortgage for the remaining bal plus the step/natural sibling 80%. How would I go about getting a mortgage for $ 143,320.00 to pay off mortgage and other interests in the house ?