Thanks Linday and Ginnyfrank, I have just sent an email to immi to clarify the case. I especially worry because my mom and dad still live in the same house, so dont know how they would consider their relationship in this case. There is no such thing called "family book" in my country, there is only a book of "household registration", so I dont know how the immigration will enquire to classify how many children they have.
May I also ask, for the 2nd instalment ($44k), how far we would know in advance if the visa should be granted, until the deadline to make the payment, because to have enough money for the 2nd instalment it would require my mom to sell one of the house, which we dont want to do unless knowing that the visa will be granted, and she has passed all the requirements.