DD will be 3 next month. I'm not sure if she will really understand when I tell her she's going to be a big sister or that there is a baby in mommy's belly.

When/how do you plan to tell your LO's that they are going to have a brother or sister?
ETA: Sorry, just saw this was already posted.
BFP 08/11/2012, DD#1 born 04/18/2013
BFP 01/01/2015, EDD 09/07/2015, CP
BFP 10/09/2015, EDD 06/01/2016, MC @ 7 weeks
BFP 03/12/2016, DD#2 born 11/15/2016
BFP 01/28/2019, EDD 09/29/2019
Re: When will you tell siblings?
We've that after our screening scan at around 12 weeks, we'll tell them.
We we intend to take them out to supper at their favourite restaurant and present each of them with a wrapped book (title yet to be determined) and a note with a copy of the scan image, since they look decidedly like a baby at that point. They've been asking for a baby sibling for years so I expect it will go over well.
Son, K, 9 | Daughter, C, 5 | Daughter, M, expected November 7, 2016