Is it normal that my sister's 15 and had a crush on a 20 year old for 3 years?

My sister met her crush on November 4 2010, and started crushing on her mid-July 2011. She still has a crush on her to this day, even though they don't talk as much. By the way yes, her crush knows she liked her in 2011, but doesn't know that my sister is still in love with her.