5w how to keep from family

Been through ivf and am now 5 weeks and 4 days pregnant. My parents and sister know as I've been very I'll in hospital with ohss. So far we've managed to keep it from my husbands side of the family, as they aren't understanding and would tell the world before we are ready to. My question is how do we keep it from them without them becoming suspicious?

My sister in law isnt the nicest of people and is dord her best to interfere as much as possible in our lives. I have a feeling she may be expecting an announcement but it's our information to share and we aren't ready to yet, at least until 8 week scan (preferably 12 weeks). I feel she is going to ruin the surprise for everyone if she doesn't back off.

Any advice?