I'm thinking of announcing that my wife is pregnant after my sisters baby shower
Why would you want to announce your wife's pregnancy after your sister's baby shower?
Since the whole family will be there for the baby shower it's a good time to announce the pregnancy.
I'm sure you're excited to tell everyone about your wife's pregnancy, but you should wait for another time. This is your sister's baby shower and it's a day to celebrate her pregnancy. Making your announcement at this time will take the focus away from your sister.
The whole family is going to attend the event so it's a good time to find out how to stop the pregnancy, why wait for another time?
As long as your not overshadowing the baby shower. I would advise to run in by your sister first to see if she would be offended by it.