If you got your wife’s sister pregnant, you need to tell her, because not only did you cheat on her, her sister has also betrayed her trust, and she deserves to know that. Additionally, since it is your wife’s sister, it’s not some baby that your wife will never meet, and this type of secret can sometimes come out in the worst ways, which will hurt your wife even more, and it would be loads better if you just told her yourself and took responsibility.