1
general::
In an advantageous position; also, financially well off. For example, The terms of the will left Mary sitting pretty. Although the use of pretty in the sense of ‘‘advantageous" is much older, this colloquialism dates only from the early 1900s. It was given extra currency as the title of two different musicals, Sittin' Pretty (M. Moore, 1921) and Sitting Pretty (G. Bolton and P.G. Wodehouse, 1924).
American Heritage Idioms
2
general::
Phrase(s): *sitting pretty
living in comfort or luxury; living in a good situation. (*Typically: be ~; leave someone ~.) • My uncle died and left enough money for me to be sitting pretty for the rest of my life. • Now that I have a goodpaying job, I’m sitting pretty.
McGrawhill's American Idioms And Phrasal Verbs