From the sounds of things, you were lonely and vulnerable and your sister's husband saw that a mile away. Maybe you've known him a long time. What surprises me is that of all the guys you could have chosen, you chose this guy - and betrayed your sister at the same time. I'm sure there's back story that justified this for you at the time.
You have some choices to make. None of them will be easy.
If you decide to keep the child, you have the option of telling your husband that you were unfaithful but not identifying your brother in law as the father. Your husband will probably be devastated. If your brother in law doesn't know you are pregnant, you can simply explain it's someone else's. How you'd maintain this story, especially when you are all together, for the next 60 years, I just don't know.
If you come clean, you run the risk of losing everyone - especially your husband. It will probably ruin your sister's marriage. Out of this whole scenario, who is likely to forgive you the fastest? Your sister. It may take a decade, but blood is always thicker than water. But she may never forgive you. I wouldn't.
The easiest solution which also inflicts the least amount of damage on everyone around you, is to terminate the pregnancy.
I have dated someone who was in the armed forces and the time apart is tough. In all the months I was alone waiting for him to come home, crying myself to sleep, I never contemplated being unfaithful - ever. One thing is certain in this whole scenario - you aren't happily married to someone you love. You betrayed your own sister to get this affection. She deserved better and in my opinion, you've been very selfish. I suspect this will be a tough lesson for you, no matter what the outcome is.