M.M.
I had a 20 month old at the time of my second and I told him 2 days before she got here she was coming (c-section) and we talked about it a lot for 2 days and he was very excited to see her when my family brought him to the hospital and they have been the best of friends ever since. They can't handle too much waiting at 20 months (at least in my experience) so I would wait until really close. The best thing you can do is get her very involved when the baby comes home. I told my son how much sister loves him and needs him and I let him help take care of her by handing me diapers and blankets and help me talk to her and that really made him feel important which really bonded them. Good luck, the first 1.5yrs with 2 that close is pretty hard but it gets much easier when the second can walk and communicate a little. Congrats and may they be best friends!