继子不到21岁,是否也可以加上?
-Yes. Ask NVC to add these two people into the process when NVC contact you.
Can step son use the following equation to calculate the child CSPA age as well?
-Yes.
He is 20 years and still at collage now. Thinking about to come here after granulate from college. If this is the case, he will be 23 years old.
-He had better immigrate to US together with the parent, and then apply for reentery permit to stay in China to finish his college study.
If my sister become a US citizen (5 years later), it means her son can get married and will not slow dwon the process? or will go different process?
-Then, your sister can ask USCIS to upgrade the I-130 from F2B to F1 she becomes a US citizen, and then from F1 to F3 after her child gets married. The priority date will be the same, but use different category.
or it means the US citizen will be very quick to bring her unmarried child to US?
-Not at all, it just uses different category with the old/same I-130. If her child gets married before your sister becomes a US citizen, the I-130 will be void (equal to that she never files I-130).