Spilling the beans!!

Alex • Married to my best friend & Mom to the sweetest little girl ever
So my sister is pregnant right now too with her 1st. She finds out what the baby is this week and i dont want to spoil her moment but Im terrified my 3 year old is gonna tell everyone there is a baby in mommys belly!! Ahh!! Any suggestions?! I'm planning on telling them this weekend if she (my daughter) doesnt blurt it out befor then!