If you're worried that marrying her is a sin you realize the quran is quite clear that once a child is conceived, it has the right to life. The Quran makes it very clear that all life is sacred. It is never permissible to terminate a pregnancy because one fears being unable to financially support a child or another child. It is God, who is the Provider and Sustainer of all life.
Furthmore Prophet Muhammad was heard to say, “A woman may be married for four reasons: her wealth, her lineage, her beauty, and her religious commitment. Marry the one who is religiously committed.”
Looking at the situation, she has a choice to make whether SHE wants to marry you, assuming she did you could rationalize it would be for her lineage and yours. However, I dont see how youre so comfortable committing adultry and simultaneous concerned whether or not it would be proper to marry the mother of your child?