We did a similar thing with announcing our soon to be son! I was about 6 weeks along when my daughter's birthday was...so we waited to tell anyone in the family, and had her wear a "big sister" t-shirt to her birthday party.
I always knew RIGHT AWAY when I was pregnant. This time...I knew about 24 hours after the conception. I literally woke up in the middle of the night dry heaving in the bathroom...and was like "oh....NO!!!!!!!!!!!!".
*You have to understand, at the time my husband was only home for a couple days each month...and he was at his new job across the state while I stayed in our old place with the kids-with our house on the market, him living in a hotel room at the time...while we were trying to sell one house and buy another, and move across the state. This was also a "birth control" baby-we had a boy and a girl, and we were not planning on having any more.*
I literally took a pg test about twice a week for 2-3 weeks, until I was finally far enough along for it to show a positive result on the test stick...which happened on Christmas.