My memory fades, of course, and the record shows that three years ago I wasn’t as good at noting developments about Siena, but I have the distinct impression that Thalia is a little more coordinated in the muscle/motor-control department. She also seems a little quicker to imitate certain physical actions.
In particular, there is a little cube toy we have that has a knob on the top you can push that causes some widget on one of the sides to spin or rotate. If you bang on top of the cube the widget can get spinning quite fast. When Siena was a baby, I’d bang on the top of that frequently while she watched, but she never really did anything with it other than pick it up and try to chew it. Thalia, after a few sessions of this, started imitating me and hitting the top of the cube to make it spin.
Another example: One of the toys Siena had as a baby was a stacking toy, with light stuffed fabric O-rings. I would put one of these rings on Siena’s head, she’d realize it was there (babies seem to have very acute sensitivity to things placed on their heads), she’d reach for it to take it off, and then she’d misjudge where it was and knock it off her head. It took tens if not hundreds of iterations of this before she was grabbing the ring on her head instead of pushing it off. Grabbing it when you can’t even see it presumably requires some understanding of object placement relative to self, or at least some degree of good luck. When I started this trick with Thalia, she reached up and grabbed it off her head, and then basically presented it to me and handed it back. I thought it was sheer luck, but when I repeated the trick ten or twenty more times, probably 90% of the time she’d grab it instead of knocking it off her head. So she’s either luckier or she’s got a better sense of where things are around her.
Or, as I said, my memory is bad and there’s no difference and I just can’t remember what Siena did. Sorry Siena!