sister in laws.. ugh

That is a really d*** move on her part.
You really should "remind" her that you told her in confidence, and wished it to remain that way.
She should know that she f***ed up, even if it wasn't meant maliciously at all.
Some people are just tarts. If you point out casually that you feel shy talking to a whole bunch of fb people about something so private in your life, then maybe she will think twice when she possibly decides to create a facebook baby shower event on your behalf.

You probably don't realize this, but 18 yr. olds don't have mature relationships - you're just a teenager! Does your SIL like your husband? How long have you known her? Relationships grow over time, as you share experiences and your roles in a family.

Wishing you well.