Just for fun, here is another comparison of the kids at around 6 months of age. It is hard to do guessing games as they wore more gender specific colors as they got older, but here they are:
Apparently both Aidan and Ella were sitting by themselves at this age but Olivia still sits with the assistance of the boppy. She can sit for a long time, I am just not completely confident in her skills yet, so she has the boppy to keep her safe. And who am I kidding, Ella was getting ready to crawl at this age.
So who do you think looks like who?