Love her, Marry you . . .

[YAX]

The other girl found out that he cheated on her with you so his chances with her are gone and if he's marrying you, then you've won. Note: you got pregnant by him because you wanted him (DUHHH!! women don't WILLINGLY fvck guys they don't like, do they?) so live with your victory and learn to be happy.  Geez! Some Women! even in victory, they have to find some way to turn it into a tragedy.   

[Gemini]

My question is mostly to the women. How would you feel knowing that your husband was with someone else and loved that person but only married you because you got pregnant? What would you do and how would you feel if you found this out years down the line?

Obviously there were some feelings involved for dating and eventually getting pregnant. Your sis-in-law should put the past behind and be the best wife she can be so he will not regret marrying. I'm sure she probably regrets it too but make the best of what they have. There's really no point in re-hashing the past.