Install new window sills with wood glue and a pneumatic nail gun.
Or, in our case add them. We had our house built and I couldn't believe it when we moved in and had no window sills and only 7 foot ceilings. This is going to be my honey do project for 2015 (eek, I have 10 front windows alone!
Window sill idea for replacement windows
widen window sills with this Window trim tutorial // Yellow Brick Home
How to Install Vinyl Replacement Windows — The Family Handyman