How to tell SIL I'm pregnant?

My husband and I have been married for 8 months but before we were even married my controlling SIL tried to make me promise that she would have the first kid. I said nothing.. I was thinking OMG! Well she and her husband have been trying for almost a year and she had voiced her opinion that we needed to wait until we are 24 until we try. What do I do? What do I tell her? Even my MIL is like we are too young. We haven't told anyone yet. He's 23 I'm 21.