Thanos and Thena
Social media is going nuts because they are cousins.
I understand that some have not read the comics but most people going to see these movies probably have. This has been part of the lore for 30 years and not a secret.
I think its the other way round
Most people who watch MCU movies are not comic book readers. IF MCU was restricted to a majority of comic book readers, then it would never have achieved the kind of success it did.
Who is Thena?
This is from the comics. No idea how they will address it in the film.
Thena's father Zuras was the leader of the Eternals. His brother A'Lars is Thanos father. A'Lars had different ideas on how the Eternals should work. Rather than risk civil war he left Earth with his followers. His wife had Thanos after they had settled in their new home.
After Zuras death, Thena became the new leader of the Eternals on Earth.