After Luke, the Jedi's and Sith defeat Abeloth, Luke comes to the conclusion that the galaxy needs both the Jedi and the Sith to maintain the balance in the force. Abeloth existence was what had imbalanced it; her destruction, I'm assuming, is what the supposed Chosen One, Anakin, would directly/indirectly lead to. So, Anakin's family/descendants leading to the destruction of Abeloth may be the fulfillment of the prophecy.
Again, Abeloth played a large part in the force psychosis that would plague force users which often led them to go all homicidal all of a sudden. Her manipulations of the force were largely confined until cracks in her prison began to appear, she then had more influence etc. This was a cycle as other stronger force entities would reseal her prison. These force entities (father, son, daughter) were immortals, celestials that are bound to the force. Apparently the chosen one, Anakin, could have power over them and with dagger of mortis could kill them or at least seem to. Abeloth was an lady that came to the planet Mortis to learn from these celestial where she became the Servant who would later do that which was forbidden her, drink of two pools of knowledge. The pools of knowledge were like the lightside and darkside wellsprings. The daughter and son were each identified and associated with one, either light (daughter) or dark (son). Upon finding the Servant corrupted, the son and daughter then set out to imprison the servant. (father probably participated as well)
father, son, daughter are about 1 Million years old
servant is 100k years old