My firstborn, who looks more like her dad, was friendlier with my in-laws than with my family when she was a baby. Meanwhile my second baby now, who resembles me more, is friendlier with my family and less friendly toward her dad's family. How strange! Can anyone explain this?

Babies have their own personalities too. They have their own likes and dislikes, their favourites, things they hate at sight, touch or taste, and they do warm up to different peope differently. It's the same for my 2 kids. My daughter at 6 months would only cry the moment my relatives come near here or attempt to carry her but she lets my maternal grandfather carry her even though he's the type of man who doesn't show much affection. My son on the other hand loves the attention of anyone and everyone and will let anyone carry him (as long as I am within sight) but cries the moment my dad, who we live with, carries him. I don't think there's any scientific reason why babies react differently to people. They're just like us. I like to thing that babies are like adults, only much much simpler, less complicated and cannot talk yet lol.