I'm going to tell my mom that my sister-in-law is pregnant?
Is she really pregnant?
Yes, but I feel she's being deceitful by keeping it from my mom.
It would be wrong for you to tell your mom that your sister-in-law is pregnant if your sister-in-law doesn't want to tell other people yet. If someone tells you something in confidence, you should keep it to yourself unless they say it's alright to tell others.
I think it's right to tell what my sister-in-law is doing, she's keeping it from my mom.
She might have a reason why she is keeping it a secret. Sometimes people have had miscarriages and she might just want to make sure the baby makes it past the second trimester before revealing it.