I want to apply for my sister's green card through family based immigration (4th preference). I am a USC. They already have a 5 yr. multiple entry visitor visa. If I apply for them, which is going to take take around ~12 years for their dates to become current, can they still come to USA using their existing valid visitor visa, or it becomes invaid? What impact would the greencard application have on their visitor visa?? Thanks for your response.