I'm sorry MumHoll. I had a very similar experience at the end of June, and I'm afraid it didn't end well for me.
I believe it is possible to miss a heartbeat on a fetal pole that is only 3 or 4 mm long. However, the size and date discrepancy, and the lack of growth, is a big worry. The nurse at our EPU told me they never say never until the last scan, because things can happen that aren't "textbook", but I too would advise you to prepare yourself for the worst.
The waiting is truly horrible, and I agree it does feel like torture. When I was in that position I went out shopping (when I could bear to) and wasted lots of time on MN, talked it over with DH and good friends (my friends didn't know I was pg until then, but I needed their support and they were happy to give it), also spent a good bit of time crying and sleeping, TBH.
Personally, I would want the support of my parents and sister. This is a happy time for them, and it is very selfless of you to want to protect them. However, you also need to protect yourself, and with a new baby around there is so much potential for you to be upset and reminded of your own worries. Knowing that you are going through a hard time shouldn't take away the joy of your sisters baby for them, but it might make people think before they unknowingly say anything insensitive to you, and will explain if you are suddenly in tears round the baby etc. Think of yourself too - don't be a hero if you do need their support.
I have my fingers crossed that everything will turn out well. Take care x