I'd give it a week or so, then ask her WHY she doesnt want your OH to know? Does he gossip within the family, is he likely to blurt it out, is there a reason she doesnt trust him?
SHe is prob caught up in the emotion of finding out, the hormones, morning sickness etc. It puts you in a very awkward position though, between your sister & OH. If you had the same news, would she be able to keep it from HER OH? I'm sure if the tables were turned, she woudlnt keep it her herself, & neither should you be expected to. I dont mean its ok to tell the whole family, but if she cant trust your OH, then I'd want to know WHY?