My MIL has a theory that couples always spend more time with the wife's family than with the husband's family (and she's very bitter about it).

For us it is mildly true, only because I tend to spend time with my family when my husband is at work. When we are all together, we spend equal time between the two. One lives within walking distance and one lives 20 minutes away.

ETA: also, who do you enjoy spending time with more? Who would you spend more time with if they were both equally convenient?