Hello,
First of all, I am sorry that you are struggling with such a hard decision. Second, If she has had her tubes tied, is the pregnancy a safe one for her? Has she been checked to see if it is a tubal pregnancy? Lastly, this is a subject that most people need to discuss with each other, how does the mother feel about keeping a baby with the odds of having the same medical history as yourself? She accepts you having it so maybe she will be accepting and willing to have a child with the same condition. Children are a blessing no matter what conditions they come in, but it is up to the both of you to want to love and care for this child. If you do not think it is possible to give the child everything he/she needs then you should prob discuss this with the mother in the most respectful and diplomatic way possible. Ultimately, it will be her choice, not that it is fair, it is just that way. Good luck to you and your family.