It is tough to find places to refi small principal loans. The minimum Amerisave.com will refi is $50,000. But I came up with a creative solution for you.
If you do a cash out refi for $50,000 (taking out 5k in equity) you can get this 15 yr loan:$50,000 for 15 year @ 3.375% with APR 3.957% and ~$1,251 closing costs. New Payment = $354.38http://www.amerisave.com/closing-cost-d ... tsOption=0
Using the mortgage calculator from bank rate.com I estimated your MIL current P&I payment at $314.49.
Amortize $45k @ 6% over the 21 year remaining period = $314.49. The TOTAL interest paid on current will be $34,250.Now take $5k from cash out and paydown principal immediately
(don't buy new furniture etc) which reduces the term in years to 13year loan paying a TOTAL interest of $10,770. That pays off your MIL 8 years faster and saves her $23,480 in interest payments.
Factoring in the closing costs it is still a savings of $22,229.
The payment is ~$40 more per month BUT that is on the assumed current $314 PI (45k @65 over 21yr). At the time the 30 loan was originated her payment was higher then $314 so the mothly increament will be less then $40 but that is all I had to go on.
Hope this helps....