when i divorced my first wife,she left and was happy to let me live in the house until she needed her half of the house value,i paid the bills and mortgage for 10 yrs,and when she wanted her piece of meat,the house was valued and i had to pay her 50% of the equity in the house at that time,it suited both of us,but i did try and get it reduced,but to no avail,your sister is very lucky she was able to pay all the bills in that time,life goes on,tell her not to be greedy and take what is on offer,move on and put it down to experience in life.