To begin with, if your birds are not at least two years old, they should not be together or have a nest box. If the pair is old enough, they should not be given a nest box until they are clearly bonded as a pair that wants to breed. The pelvic gap is not a reliable sign at all. Males can have a wide gap and females can be so close you can’t find a gap. It’s an old fashioned method of sexing, that some swear by, but with a 50/50 chance of being right, this is why it can seem to be a valid way to check. Most cockatiel mutations can be sexed visually based on feather markings. You can send me some pictures to firstname.lastname@example.org and I can let you know what mutations they are and how to tell what sex they are. It sounds like you may have a normal grey and a lutino and if that’s the case, they can be visually sexed as long as they are a year old or older. For now, I recommend that you remove the nest box. Otherwise the pair is going to fight over it and will never bond, which means no eggs or chicks.
Thank you for asking Lafeber,