I'm going to tell my mom that my sister-in-law was pregnant
Why would you share that information with your mum?
My sister-in-law is 19 and she's an idiot, she shouldn't have the right to decide the time to tell her parents
When you meddle in other people's business, you are involving yourself in a situation where the outcome doesn't fall on you. Let your sister in law tell your mum when she feels the time is right, not you.
I don't care since I want to hurt her and make her feel guilty
What do you have to gain by making her feel bad? Acting out of a place of harm is no way to interact with family. You should talk to your sister-in-law about bringing up this topic on her own terms.