How old is Alexandra in Part I?

In Part II, she is 40 and Carl is 35. It is confirmed in this part that Carl is about 5 yrs. younger than Alexandra. In Part II, 16 yrs have passed since her father's death. That would suggest she is 24 yrs. old in Part I.

However, in Chapter 1 it says that Carl is 15 yrs. old. That would make Alexandra 20 yrs old in Part I.

How is this discrepancy explained? Am I missing something? Did 4 yrs. pass betwen Chapter 1 and Chapter 3? It seems like her father was on his deathbed at the end of Chapter 2, so that doesn't really make sense to me.