Hi.
As you know some characters swear by the old gods even if they follow of the Faith of the Seven and even if they don't have parents with different religions.
According to the book "the World of Ice and fire" The Andals who brought the faith of the Seven with them gradually conquered the south of Westeros. Seeing the old gods as little more than demons, the Andals destroyed the great white trees wherever they found them
Catlyen Stark is a follower of the Faith of the Seven, in her chapter it's written :
"Perhaps she would go to the godswood tonight and pray to Ned's gods they were older than the Seven"
Also Brienne of Tarth and other characters swear by the old gods with the new gods even though they follow of the Faith of the Seven
I don't understand these characters, they don't have parents with different religions
in the case of the Stark children I understand they have a father who worshiped the old gods and a mother who worships the Seven but why would other characters such as Brienne swear by the old gods and the new gods even though she doesn't have parents with different religions and why would Catelyn pray to the old gods when her religion consider the old gods demons? Catelyn don't have parents with different religions.
Nearly all people in real life who believe in two religions have parents with different religions.
Sorry for the mistake instead of writing some I wrote a lot. Thanks for your answer.