A recurring idea in the Souls games is of people holding on to honours and duties forever, without interrogating whether or not those ideas should still be held. For Sif, protecting the grave of Artorias is the most important thing left in the world, and while Sif definitely has reservations about fighting you, this duty must be upheld.
Was Sif right? That's a more difficult question. By protecting the Covenant of Artorias, Sif was preventing anyone from defeating Gwyn, therefore leaving the world in a stale state, impossible to move forward. But the two possible endings for Lordran aren't particularly great. Linking the fire just delays the inevitable, and becoming the Dark Lord just lets Kaathe and the serpents abuse the power of the dark once again, just as they did in Oolacile and New Londo. It feels like a game with no winning, no solution.
