Sometimes I see things on MN and, as the mum of three boys, I think ‘I will learn from this, and do my best not to treat any future DIL this way’ - but this is not one of those occasions - NOT, I hasten to add, because I would do what the OP’s PIL have done, but because it wouldn’t even occur to me to share their news unless and until they told me I could!
I hope that, when the time comes, I will have as good a relationship with my DILs as I had with my own lovely, late MIL - I couldn’t have asked for a better example of how to be a wonderful MIL, and I miss her so much. If I am honest, I had a better relationship with her than I have ever had with my own mum, which is sad.
I do understand how exciting it is when grandchildren are born, and I am sure I will be beside myself - but that would be no excuse for overstepping boundaries. No matter how excited I am, I will still remember that I am the grandmother, not the mother, and that puts me in a very different position. And I wouldn’t expect to be as close to my DIL as her own mum is - that would be madness.