announcing to mum

Joelene
Anyone have any ideas on how I can tell my mum I'm pregnant again? 3rd baby (4th pregnancy)
I am 12 weeks and when this Bub is born my son will be 20 mths and daughter will be 7. 
I have told my dad and hubby has told his parents and friends. 
I haven't told my daughter yet because she can't keep secrets. And although she doesn't know, she talks to my tummy already. But hasn't said anything to anyone. She told everyone when I was pregnant with my son and even before I was pregnant with him. Not sure whether I should tell her first so she can tell mum or wait and tell mum myself.. Kinda scared.
Any ideas that would make it less scary would be appreciated. 
Thanks