Linda Besner explains the findings of a study suggesting that your mom's parents are the better half:
[Study authors Antti O. Tanskanen and Mirkka Danielsbacka] found that when one or both maternal grandparents were involved in a child’s life, the child’s behavioural and emotional problems were likely to be lower than average. When it was an equally involved set of paternal grandparents (or a single paternal grandparent), the involvement didn’t correlate with any diminishment of the child’s behavioural or emotional problems.
The researchers explain why this is consonant with contemporary theories of human evolution. "Difference between maternal and paternal grandparents has been explained with different reproductive interests: a daughter is not replaceable but a daughter-in-law is."
(Harsh words, kind of. I don’t have in-laws, and I’m not sure I want any if this is what’ll be going through their reptilian brains.) "It is in the paternal grandparents’ fitness interests to exploit the fertility of the daughter-in-law. By contrast, it is in the maternal grandparents’ interests to increase their daughters’ well-being for example by lengthening the birth intervals which can increase the survival probability of her already born children. Thus, maternal grandparents and matrilinear kin in general are expected to increase child survival more than paternal grandparents and patrilineal kin (Euler, 2011; Mace & Sear, 2005)."