:( im so sorry that you are in a position that you cant share this happy news without fear of the negativity. :(
first of all, i would wait until you start to show and/or you are 12 weeks along (after the chance of miscarriage goes down.
then i guess pray. obviously, you cant not tell them, and i dont know if things would be any better if you didnt tell them until it was unavoidable, cuz then they might wonder why you didnt tell them, and i guess you know the answer to that, but do you have the strength to say that? i dont know.
i guess i would just keep it simple and to the point. and then move on i guess. do your best to have a "change the subject" topic in mind for times when she starts ragging on you for something that no one can change
and you know, it goes the same about the 3rd child as it does for the first: if you wait until you can afford it, you will never afford it. a friend of mine recently told me, if you want another baby, do it! then worry about the other things later. money comes and money goes, but the love as a family you will NEVER regret adding to!!!