What Is The Remedy For Irregular Periods?

I have irregular cycles that I'm working through by taking Fertalaid. My doctor was unable to find an issue that would explain my irregularity or lack of pregnancy. He told me to lose weight. I've managed to lose a bit of weight over time but I need a better explanation. Especially when I have a morbidly obese sister in law who seems to be able to have children without an issue.