While these people may indeed be able to offer relevant advice, because they have a strong connection to only one side of the couple, the other person may perceive favoritism or feel that the help is biased.
Favoritism becomes a factor when some parents find it much easier to relate to a child with a flexible temperament or one whose temperament matches their own than to a child who does not fit in as well.
If you're dating a boss or another person who is higher up the career ladder than you, there could be bouts of favoritism that other coworkers won't appreciate.
Before starting his career, the nephew of Francis Ford Coppola, Cage changed his last name to avoid favoritism in his career due to his famous relative.
It is also important for parents to avoid overtly comparing their children to each other, and every effort should be made to avoid favoritism.