Their second child, Jia Li, is probably more used to the hometown culture than the Asian culture. You said the first child eats almost all her food with chopsticks, which says to me she sometimes uses a fork. I'm sure Jia Li probably sees this and favors the fork over the chopsticks.
I have noticed that my Sims will use the culture from the country they traveled to for several weeks and slowly resort back to their hometown culture. If they were to travel again to that country, they pick up the customs much faster.
As a side note, have you noticed the parents using a fork occasionally?