Announcement Etiquette

I'm 4 weeks pregnant exactly and my sister just told me today that our stepsister is also 5 weeks pregnant! Stepsister is planning on telling the rest of the family at Christmas, so when should we announce that I'm pregnant too? I'll be 12 weeks Jan. 16th- is that too soon after her announcement? I just want her to enjoy her moment. (BTW it's Baby #2 for both of us)