Does Lorick #49 bedevil you? Read it... and weep!

I'm a lady, both cunning, and fey;

Femme fatale? Yes, I curse and betray...

Stole the scabbard and sword

From my half-brother lord...

With enchantments I lead knights astray!

Hint:

My sister and nephew and me, we are biting and deadly - all three!