Strengthen genes by breeding the desired trait (siblings). By strong and stable genes, I mean they will breed true. If you can find/buy similar trait, do so when introducing new genes. But your line should be stable enough to produce desired traits, thus you wouldn't have to repeat the whole process.
It is believed (in my area) best to use the female from your line to cross to new genes. Fry inherits more of mom's form thus you should still produce desired traits. Further, quality females are harder to find compared to males :)