My guess is that the two Asian students are from a country where dual-citizenship is legally restricted (since around the time of independence from some European colonial power) such that at around age of majority they have to legally make a decision about which citizenship(s) they are surrendering. If it is not that, then it's probably a decision based on a financial burden or financial reporting burden consideration. These sisters or their family probably intend to have them return to live outside of the US after finishing their schooling in the US. Quite common for members of the global elite who are of some Asian or African national origins.