Dear @someoneingermany,
Welcome back to the Wefugees Community and thank you for being such an active member.
With holding a residence permit according to 19d, family reunification is possible if a number of requirements are fulfilled. However, exept from severe hardship cases (very difficult to get accepted), only members of the core family have a legal right (if fulfilling the requirements) for this type of visa/residence permit. This usually includes the husband/wife and minors. In the case of the 19d, the marriage should have taken place before the reference person moved to Germany. Otherwise, the decision will be at the discretion of the immigration office. Please see, for instance, the website of the Refugee Council Lower Saxony for more details.
Regarding your husband's nephew, could a study visa according to section 16b of the German Residence Act be an option? Many applicants are struggling to save enough money for the required so-called "Sperrkonto" and I am sure your husband's nephew would be very happy to receive some support here. For more information, you may want to check this website, for instance.
I hope this helps and please don't hesitate to get back to us with any further questions.
All the best,
Meike