I have a 15 month old son and another little one due in October. Any advice/suggestions on how to make the transition easier for my son once our new addition arrives? :)
First of all, congratulations! The more the merrier. :) It sounds like your little ones will be about the same distance apart as mine. As far as preparing Caleb for the transition, there wasn't much that could be done as far as trying to explain it. He would say, "Baby in Mama's tummy!" but he really had no idea what that meant. I just tried to get him used to what it would look like having a baby around. I put a doll in the car seat carrier, the swing, etc. I still don't think it really mattered much to him, but at least I was trying!
There was one thing we were very resolute about and that was celebrating our last weekend with Caleb before his sister arrived. I was induced, so I knew when the baby would be born, but this would work even if you had a general idea of your due date. We took the whole weekend to celebrate Cub and prepare the house, together, for the new baby. We baked cookies with blue and pink sprinkles, to celebrate both baby sister and big brother, and Caleb happily decorated them. I made a "Welcome Home, Nomi!" sign and Caleb "painted" it with finger paints (more like smearing than painting, but it worked!). I still have that sign tucked away in Naomi's nursery. Then we took Caleb to the zoo and out to this really fun pizza place for dinner. The whole weekend was about having fun and being excited about the changes to come and I'm so, so glad we chose to do that.
As for the actual transition of having a new sibling, that's just a work in progress! Every toddler is different with every baby. Some toddlers are immediately in love with their new sibling and want to hold them, feed them, etc. Cub wasn't quite that enthusiastic--he liked Naomi at a distance, but that was it. But, when he realized she was here to stay, he started warming up to her. Now he misses her when she's gone and he's definitely taken on the older sibling role of showing her how things are supposed to be done. :) My advice for the actual adjustment stage would be in two parts: Firstly, when the new arrival is an infant, let the toddler "help" you by grabbing diapers, throwing diapers away, hunting down pacifiers, etc. The more they feel included with the new baby, the better. Secondly, when the kids are old enough to start interacting, give them time to do it alone. I try to give them some time to learn to play together on their own, instead of rushing in to moderate as soon as I hear a squabble.
Don't get me wrong--you'll need to teach your toddler how to play, too! It's important for toddlers to learn how to play with infants and it's important as the Mom to be sympathetic to both parties. Caleb should learn to share, but it also stinks when your little sister invades your space and messes with your stuff. I've taught Caleb that when Naomi takes a toy he wants, he should gently take it back and give her another toy in its place. She doesn't really care (yet) and it teaches him how to be proactive with her, rather than simply defensive.
I hope this helps! Congrats again on the little one, how fun!!
Have a lovely day. :)