While they aren't the longest of the family relationships in the Marvel universe, the most convoluted family chain must go to the Summers/Grey clan.
Major Christopher Summers (known as the leader of the Starjammers, Corsair) and his wife, Katherine Anne Summers had three sons; Scott Summers (Cyclops), Alex Summers (Havok) and a brother who was born off-planet after Corsair and his wife were taken prisoner by the Shiar. Gabriel Summers, later called Vulcan would for a time, rule the Shiar Empire. All of the Summers clan turn out to be powerful mutants.
Scott Summers (in at least some parallel realities) marries Jean Grey (Marvel Girl/Phoenix), one of the world's most powerful psykers (a telekinetic and telepath) to give birth to several potentially world-shattering children/descendants:
Cable (Nathan Christopher Charles Summers): Son of Scott Summers and Madelyn Pryor (a clone of Jean Grey; don't ask...)
Nate Grey: is another alternate version of the regular Marvel Universe hero Cable, hailing from the "Age of Apocalypse"; his parents are Scott Summer and Jean Grey from his timeline (Earth-295).
Rachel Anne Summers: She is the daughter of the alternate future counterparts to Cyclops (Scott Summers) and Jean Grey-Summers from a harsh dystopia, the "half" sister of Cable, a niece of Havok and Vulcan, and a powerful mutant in her own right.
What makes this clan so problematic is they have all existed inside of the canon Earth-616 timeline simultaneously even though they all hail from different parallel realities and different time periods. In their realities, they were so powerful they disrupted the timestream sufficiently enough to travel from their future to the main canon universe.
Other notable families include the Inhumans who have long bloodlines which trace the royal family back generations. We are introduced to the what we know as the familiar royal family first seen in Fantastic Four #45 (1965). This lineage was lead by Black Bolt and Medusa until his recent and untimely death.
From left to right: Crystal, Medusa, Triton, Gorgon, Black Bolt, Maximus, and Karnak
Inhuman Royal Family Tree from Official Handbook of the Marvel Universe #5
I have avoided the divine beings in the Marvel Universe such as Thor, Odin, Zeus and others because the Marvel Universe has appropriated the mythologies of numerous cultures and their often twisted and extended genealogies.
- Since the Marvel depictions tend to resemble the mythologies, it is safe to assume most of them include their creation myths and their primal entities. Marvel has even created a system of Elder Gods who MAY have been the progenitors to some of those deities. Thor Annual #10 (1962) took a stab at explaining where Gods came from...**
- The Elder Gods are incredibly powerful ethereal beings with vast cosmic powers, who are the survivors of the first wave of entities that were spawned by the Demiurge visiting Earth soon after its formation, 4 billion years ago. Gaea currently resides within the Earth itself. Oshtur formed the Vishanti to assist Earth's Sorcerer Supreme. Set and Chthon were driven from Earth by Gaia's son Atum, but still exert influence through their followers and artifacts.
- According to these panels, it was the interaction of the minds of humans which helped to spawn all of Marvel's deities, but Marvel has gone back and forth on these concepts so it is best to consider them apocryphal and legendary without anyone capable of confirming them.
The Marvel Skyfathers of all of Earth's pantheons debating the impending threat of the Celestials in the famed Thor #300