We found out we were expecting our third, only a few days after my SIL found out (and announced) her first pregnancy through IVF.
So we decided to wait awhile before telling family. She ended up m/c at 8 weeks and we'd planned to tell family at 9 weeks at DS2's birthday. So that got put on hold. Meanwhile I had a 2 week bleed and thought I'd lost the pregnancy. I got really panicky that I'd have to tell my mum and sister about a m/c when they didn't even know about the pregnancy. I was so upset at the thought that they wouldn't have been able to share the excitement with me and yet, I'd want their support if I had m.c.
Turns out everything was fine and I told my family immediately after the scan to share the news. We held off telling my DH's family as it was too soon after my SIL's m.c.
Needn't have worried though, cos MIL had blabbed to everybody that I hadn't drank wine at a family get together...

So I guess you need to work out with your partner, who (if anybody) you want to share your news with early. If I had my time again, I would have told my mum and sister much earlier. Stuff family politics and wanted to 'do the right thing' and tell both families at the same time!