Is marriage the only way an immigrant can come to the U.S.?

My sister met a guy on vacation late last year, he lives overseas. Last month we went to visit him, where he proposed. I’m not too happy about this because my sister has only been in his presence 3 times. My mom, sees no problem with this. Yes, I know that love at first sight is an actual thing, but I have many concerns. For example, he has an infant baby, yet he is willing to leave and move to America (I believe he has joint custody).

Don’t get me wrong he is a nice man, but no one in my family knows him except my sister, mother and I. I really want my sister to sit and think about this. I feel she is moving way too fast. I think if he would come here first and my family and I could get to know him better without them getting married, we would feel more comfortable. However, I don’t know how visa’s work, except for the fiancé visa, which is what they’re planning to get. Is there a way for him to come here without them having to get married first?