Help!! Need advice.

DD is approx 12/15/16. First pregnancy, we don't want to tell anyone until after the first ultrasound 5/10/16 - I'll be approx 8+5 then. On 5/20/16 we will head back for my nephew's 2nd bday party on the 21st. My whole family will be there and I am thinking we will tell them then. Is it okay to tell them then? I don't want to take away from my nephews party, so we may tell the family the night before his party? Any suggestions?