THE CLONED STRANGERS
Not everyone would envy young Lord Miles Naismith Vorkosigan, even though he had formed his own mercenary fleet before attending the naval academy, and even though his mother was the beautiful Cordelia, the ship captain who has taught the Lords of Barrayar much about the perils of sexism. Even the fact that Miles is third in line to the throne and personally owns a major chunk of his home planet would not tempt any normal person to change places with him.

When assassins came to rid the world of his father, his mother, pregnant with Miles, was in the line of fire, and Miles was but an egg for the omelet in an all too literal sense. Thanks to heroic medical intervention, Miles survived his near fatal brush with war gas-as a pain-filled dwarf with bones as weak and brittle as some malign composite of chalk and glass. Miles is often mistaken for a mutant by his mutant loathing countrymen.

But there is one who does envy him, who wants to be him: his brother, his cloned stranger formed from tissue stolen from Miles when he was a child. For Mark Vorkosigan was created and raised up for only one purpose: to become Miles, to murder and replace him. In Brothers in Arms that conspiracy was routed and Mark made more or less compliant to his new Miles-less fate. But in the intervening years Mark has learned that without Miles he is . . . nothing. The new and better Mark doesn't really want to kill his brother, but still it may come to that: Mark to stay, Miles to go. . . .

(From Goodreads)