I'm going to lie to my sister in law about my pregnancy
Why haven't you discussed this with her?
I don't like to discuss things like that
You shouldn't lie to your sister in law. Hiding your pregnancy is wrong. She will find out eventually.
So? Then she will tell her sister, and they will talk about it
If that is inevitable, then why add the drama of lying and having them upset.   You should have everything out in the open.