1. 2 years of age is when most 'working' (as in outside the home) parents go back to work by. It's when kids start to benefit more from socialization and when my work places start to clamp down in post mat leave options.
2. Does the government pay you to stay home and home school?
With 1 in mind, if the answer to 2 is yes I can see where your sister is coming from. As society has sort of set that expectation that by the age of 2 parents are back to work or getting on with their self funded stay at home career.
I still think your sister sounds like she was being a bit of a cow about it. Maybe jelous?