A passage from a review of a new biography of Marx's intellectual partner, Engels:

He had always been an involuntary factory owner. Without agreeing to tend his German father’s business interests in Manchester he would have lacked the income for himself and Marx to live in the comfort they took as their right. The profligate Marx was constantly on the edge of penury. Engels counted his pennies (or rather his tens of thousands of pounds) more carefully but did not stint in his pleasures. He rode out regularly with the prestigious and costly Cheshire Hounds. He drank wine of quality and ­Pilsner beer in quantity. He treated himself to bevies of young women, including prostitutes. He dressed in fashion.

Engels kept up bourgeois appearances by holding his capitalist and communist lives separate. The frock-coated German industrialist bought a second home in Manchester where he installed his fiery Irish mistress Mary Burns and welcomed his socialist comrades. Mary’s sister Lizzy took her place as his lover when she died.

A possibility: In that particular social space and time, having sex with the man who had sex with your dead sister was not troubling but practical. Mary and Lizzy look alike. And because they grew up with (or were conditioned by) the same social pressures (parents, family, friends), the two are likely to desire the same type of man. That is one way of looking at it. Another is to see it as a sense of duty to the dead. It's a sense or form of love that diminished to nothing as our society evolved and became more positive. Lizzy was expressing a love for her sister by loving the man she once loved. Her body filling in a physical and emotional absence. But the love is not for Engels but for her sister's love of him. The difference, however, is almost imperceptible.

The grave does not have this kind of power on us. The dead in our world are dead and gone. A writer of the present moment could not speak of the dead in the way, say, Ibsen did in his moment, which was Engels' moment, the peak of the 19th century:

Mrs. Alving. I will tell you what I mean by that. I am frightened
and timid, because I am obsessed by the presence of ghosts that I
never can get rid of,

Manders. The presence of what?

Mrs. Alving. Ghosts. When I heard Regina and Oswald in there, it
was just like seeing ghosts before my eyes. I am half inclined to
think we are all ghosts, Mr. Manders. It is not only what we have
inherited from our fathers and mothers that exists again in us,
but all sorts of old dead ideas and all kinds of old dead beliefs
and things of that kind. They are not actually alive in us; but
there they are dormant, all the same, and we can never be rid of
them. Whenever I take up a newspaper and read it, I fancy I see
ghosts creeping between the lines. There must be ghosts all over
the world. They must be as countless as the grains of the sands,
it seems to me. And we are so miserably afraid of the light, all
of us.