Select breeding(to rat breeders please)

i've heard of rat breeders breeding brothers and sisters to make certain characteristics stronger (which- if done wrong, can make things weaker too) but i also heard that doing that will cause health or temperment problems.

so i was wondering if its safe to do that (and also breeding mother/son or father/daughter) for the rats' health

In the case of a good breeder who knows the family lines very well, this can be okay.

Inbreeding by itself doesn't cause problems. In order for problems to crop up, these problems already have to be present in the family lines. If a breeder KNOWS that their lines are free of issues, doing a close breeding like that shouldn't cause any issues.

Breeding a random male and female rat with no knowledge of their genetic background, though, is asking for problems.