i told heaps of people via sms - it's impersonal to a degree, but it got me out of talking to people, having the "but you told X first" convo...

not a single person whinged - if it's just your sisters, i'd do an sms saying "hi girls, just wanted to let you know at the same time, we're expecting another bubba in September - and we're so happy we just had to share!"

put it on them from the get go that YOU are happy - their opinions don't matter

congrats again and good luck with your pg

BG