Wow, what a position to be in!!! First of all, congratulations on the pregnancy, my heartfelt belief is that no matter what, a baby is something to be celebrated and is never bad. The CIRCUMSTANCES might not be ideal, but the baby is a miracle to be celebrated!!!
Now what to do with family depends heavily on the family situation. I, for one, would not be able to keep it quiet AT ALL, but that's because I'm in a situation where a baby is a welcome addition (perhaps not quite a "planned" addition, but a welcome one still!) and everyone in the family was happy with the news (even the ones who knew it wasn't planned).
If you are holding off waiting until the ultrasound just so you have a picture to show to make the announcement more fun, then go ahead and wait, I imagine that your ultrasound is scheduled very soon??
If you're waiting until the ultrasound to make sure the baby is healthy, then that's up to you and your beliefs and your state's laws. If baby is not healthy, would you consider adoption? Would you consider termination? Can you even terminate this late? (I have no idea what abortion laws are) If you chose to put baby up for adoption or terminate, would you still tell family? If those are the kinds of decisions you're potentially facing, then perhaps choose one or two close relatives or friends who can be a support system for you, but hold off on telling the rest until you know for sure what you're doing.
If you're waiting until the ultrasound because you just need this time to yourself to come to terms with a late-discovered pregnancy, then that's perfectly reasonable. I know for me, seeing the baby on ultrasound somehow makes it feel so much more "real", so I could completely understand if you wanted to just see the baby before you announce it.
When you do announce it, your attitude will determine their reactions. If you announce it as a wonderful miraculous thing that you are SO thrilled about, they are more likely to follow that lead and make it a positive thing. If you announce it sheepishly, guiltily, or come across in any way negative, then you are more likely to get those responses back. Do you have a partner with you right now? If so, get him on board, agree on your thoughts on this, and do it together. Say something like "we have shocking news...we just found out we're having a baby in 2.5 months!! We weren't expecting it, but we're thrilled!!!"
I, for one, am very happy for you! I hope that your ultrasound goes well, and that the rest of your pregnancy is as uneventful as the first half obviously was!!!!