you could put a "big sister" shirt on your little girl, and either have her just casually wear it at a family gathering, or send it in a picture.
you could send "grandparent" frames (or just a regular frame) to family as a gift with either just the sonogram of the baby, or with a picture of your little girl on one side and the sono on the other.
you could send flowers or a cookie bouquet or something like that... maybe for valentine's day (if that's when you're wanting to tell) that say, "happy valentine's day! love, T., your hubby, your daughter & baby." you could so the same thing with a card...
maybe these ideas will at least inspire something else if you don't like these! : )
congratulations! i pray your pregnancy is wonderful and that you are really able to enjoy these precious days with your family!
blessings,
S.