You can get your MC to SSS, you just have to breed with her a lot, and my theory is that doing so with better statted partners works the best. If your trying to get B ranked offspring and your MC is CCD, then breed with mates that have D on aptitude and eventually youll get a CCC MC. Once you have that, you breed with mates that have Cs and your MC will get a B somewhere. Then you use your MC to breed again and you'll get your offspring B.
Basically, improve MC by breeding with similarly statted nephs, then use her for offspring of her stats, then use that offspring to generate more offspring... There's always going to be something that has a similar stat on one skill that can improve your MC.