So if we decide we want to get a fiancée visa that lasts six months until we get married and after that they would become citizens or would be able to apply for IRL ?

Hi I am a british citizen and a US green card holder living in the US I am currently planning on moving back to the U.K permanently in a few months time. And I would like to sponsor my girlfriend and her daughter who are US citizens .But we have been togeather for about six months so it possible for...