Hi L.,
We have a daughter who was 2 at the end of August and I'm due with twins July 1st. We told our daughter already. Part of the reason was, I have been EXTREMELY sick. The other reason was my daughter is very active and often likes to 'bounce' on my belly when we're playing on the floor or something. So, we thought it best to tell her so she knows to be gentle with mama. It's also been nice because after being sick in the bathroom, she'll often come up and give me a hug and kiss to comfort me. :-)
The thing is, kids pick things up. Try as we might to not discuss certain topics in front our kids, we slip. They pick it up and can feel left out. I talk to her about how the babies are VERY small right now and have to grow big before they will come out. She loves me to read the 'I'm a Big Sister' book and talks about how she will hug and kiss the babies.
We don't talk about it every day, but a few times a week. I think it's important to have her involved throughout the whole process. Regardless, no matter how hard we try to prepare her ... it IS going to be a HUGE change. No longer are Mama and Papa going to be able to give her our sole attention. She's going to have to share, and that is difficult. I just feel earlier is better. She has longer to prepare, she knows what is happening to Mama and doesn't have to be afraid, and is involved.
Congratulations on your pregnancy!