Sometimes you're not sure if you're missing someone, or you're just out of your normal routine. So here's a tip: if you find yourself awake at 4 in the morning googling what the periodic table of elements looks like in the native language of the country you're currently in? You miss your sister. If you find yourself watching a documentary about auditions at a Russian ballet school? You miss your sister. If you find yourself constantly looking at the time to calculate the time in New York? You miss your sisters, and are trying to guess exactly what they're doing in that very moment. There IS such a thing as too close.
*For the person who asked, the book was The Test, by Patricia Gussin. It was actually really good aside from it incorporating 9/11 into the storyline.