The misnomer that sibling rivalry is a thing of the 'past' history- wh

The misnomer that sibling rivalry is a thing of the 'past' history- where brothers slit each other's throats in a gory race to kinghood - can and does not seem appalling; but still does.

While reading the question stem, we see that a portion of the sentence is dashed. This is called an "em dash" or a "dash" and it replaces commas, semicolons, colons, and parentheses to indicate added emphasis, an interruption, or an abrupt change of thought. The info provided between the dashes gives additional information about sibling rivalry. For convenience, you can ignore the same. The construction is now as follows.

The misnomer that sibling rivalry is a thing of the 'past' history can and does not seem appalling; but still does.

Further breaking it down (or rather expanding it - by placing the omitted verb + object)

The misnomer that sibling rivalry is a thing of the 'past' history can seem appalling, and does not seem appalling; but still does.

A. can and does not seem appalling; but still does.
can seem appalling and does not seem appalling - doesn't make sense.
B. can and does seem appalling; but still does not.
can seem appalling and does seem appalling; but still does not - doesn't make sense again
C. should and does seem appalling; and still does.
should seem appalling and does seem appalling; and still does - Good enough.
D. can and do not seem appalling; but still do.
can seem appalling and do not seem appalling - doesn't make sense again
E. may and does seem appalling; but still doesn't do so.
may seem appalling and does seem appalling; but still doesn't do so - wordy (after semicolon) and doesn't make any sense.