Pregnancy reveal for siblings

So, I have 2 bonus daughters (15 & 6) who do not know about this pregnancy yet. does anyone have ideas on how to tell them? And how to include them in our announcement? They've both been asking for a sibling so they are excited and would want to be involved. A lot of the ideas I've found are for Baby # 3 but tehinically this is my first baby. I want to make sure they feel involved but don't want to cross any lines. Thanks!