It's always hard to distinguish love and lust. Esp in usa where you have least contact with alike people. But pls think thousand times before jumping into this formal relationship. Carnal desires are common for opposite sex. Just try to arrange somebody for your sister and let her do it for you ( for marriage). In this way the problem might get solved. After a long time you may think of this as 'bachha bachhi ma bhadakuti khele jasto'. I don't know about the morality, as it depends on the culture you are grown up into.
Being social being you should think about the stigma that comes to you and your child, if this occur.
Just get married with somebody else and let her get married too.

Best of luck