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?

no mommy, i definitely do not think that there is any link between your babies liking a particular set of grandparents just because they resemble that side more. try and get your children to spend more time with both sides of the family. as they grow more familiar, they will start interacting more :)