I am an Spanish citizen married to a Chinese woman. We both live in New York City. We are going to Barcelona in june of this year. I want to invite my wife's parents to visit us in Spain so we can all see each other. We are planning to stay in my mother's house. My mother is going to apply for an invitation letter for my parents-in-law, but I heard is too difficuly. Do you have any suggestions? Thanks! Francisco and Tong