Wikipedia says (and right off the bat I must apologize for the "Wikipedia says" lede) that Byron's Manfred is what they call a closet drama, that is, a poem written like a play, but not meant to be performed, in Manfred's case because it would be the worst play of all times.
First of all, it's the kind of play where boons are granted -- actually, they're not granted, but Manfred, our Byronic hero, is too good for them:
Yeah, we love you too, buddy. Maybe we don't want your spirit walking with our souls -- did you ever think of that?
The introductory note to this play in the HC psyches us up with this paragraph: "For dramatic writing Byron was not favorably endowed. His egotism was too persistent to enable him to enter vitally and sympathetically into a variety of characters, and the hero of his plays...is usually himself more of less disguised." That explains why Manfred seems to have slept with his sister:
Oy, and I go on to add, vey. How could someone who wrote this:
Man, being reasonable, must get drunk;Write this:
The best of life is but intoxication:
Glory, the grape, love, gold, in these are sunk
The hopes of all men, and of every nation;
Without their sap, how branchless were the trunk
Of life's strange tree, so fruitful on occasion:
But to return,—Get very drunk; and when
You wake with headache, you shall see what then.
I guess it explains why he drinks.