Of course every child is different and should not be compared. But you just CANNOT help it when you are a parent of 2 kids.
If you are a parent, I am sure you must have read baby books that all start with "around this week your child should be [fill in blank]". Well, Sophie was Little Miss Average. Everything that she needed to do developmentally, and for weight/height she was generally at the 50% line. Except for her language development which was amazingly ahead of the bell curve, she did all the things that a "normal" baby (is there ever such a thing?!) should.
On the other hand, Alex seems set to be Baby Kinder-surprise. She could hold her head erect for a couple of seconds when she was born, and even when tiny, she could turn her head and track moving persons across the room. Unless you are in constant contact with newborns, that doesn't seem particularly surprising, but according to the received wisdom, "average" babies aren't supposed to do that until a little later. She wears clothes for 3 month olds (i.e. the labels read 3 to 6 months).
Also utterly normal (but truly annoying) is Sophie's attention seeking behaviour right now. This is firstly, a function of being an almost 2 and a half year old, secondly, we have some new ayi changes in the house and thirdly, a reaction to being displaced from being the center of the known universe to having to (gasp!) share the parental attention. So she alternates between testing the limits and being a whiny pain. It drives me completely nuts sometimes.
One of the things which is understandable but really annoying is her insistence that she (Sophie) is a BABY also, and wants only the good part of being a baby. This includes, but is not limited to (1) being carried around instead of walking, (2) un-toilet training herself and (3) not speaking and saying "wah wah wah" in imitation of Alex. I know she's doing it to get a reaction, so I just play along and refuse to fuss about it.
The only blessing I think is that Sophie adores Alex and she has never taken out any of her frustrations, or shown any aggression towards Alex (YET!) Her "wanting to be a baby" phase seems to be part of her sisterhood bonding experience - sort of...