Is there a proverb/saying or idiom for insiders are always undermined and treated badly and outsiders and guest are treated unfairly well. Like in trivial dispute you wouldn't believe or treat your child justly over a guest or a neighbor's child. You tend to undermine your child and overrate your child's classmates.
The tendency to devalue those who are familiar and close is expressed in Mark 6:4 :
A prophet is not without honour, save in his own country.
'Familiarity breeds contempt', is another way of expressing it.
1.proverb - extensive knowledge of or close association with someone or something leads to a loss of respect for them or it.