The rules of “proper” Japanese is that one should address one's relatives directly with “お兄さん”, “お母さん” and such, but use “兄さん" and “母さん” when speaking to others about one's relatives, and use “お兄さん” and “お母さん” to refer to their relatives. This is because in theory using the “お” elevates the status, and using that on one's own relatives when speaking to outsiders is supposedly rude, but one is not speaking to an outsider when speaking directly to one's own relatives, where one can elevate them again.
In practice, Japanese people completely ignore this and use “お兄さん” all the time to refer to their own relatives when speaking to others.
Another “rule” is that “父” and “母” can only be used for one's own parents, and “お父さん" and “お母さん” must be used for the parents of others; but again, Japanese people do not follow these rules and they're quite happy to use these terms for the parents of others. In the strictest etiquette even, one cannot use “お父さん” or “父さん” ever on one's own parents and must use “父”, which is again ignored. Even stricter is that not even “(お)父様” cannot be used for one's parents and “父上" must be used, and as rare as “(お)父様" is outside of fiction, “父上" is even rarer in actual usage.