A court can impute income to a part time worker if its demonstrated that she is underemployed. A court will look at all the factors to determine whether income should be imputed. Does she have a degree that would enable her to get a full time job at better pay? What was she doing those 26 years that kept her at a part time work? And now that 26 years has gone by, is she even capable of fulltime work at this point?
A court can also impute her current hourly wage to fulltime.... So for instance if she makes $10/hr for 20 hrs a week, the court can keep the $10 figure, but impute her hours to 40. Again, the question is whether she is voluntarily underemployed, and the ability to gain full time work.
There are no guarantees what a judge/commisioner's decision would be. But you are on the right track.