Combining my wife and daughter into a single body.

Several years ago, your mother got sick. A long, slow disease. Ultimately fatal. The family watched as she slowly declined, regularly visiting. It was painful, but all we could do. Treatments could only delay the inevitable, and it was not likely she would ever leave the hospital again.

That's when one of the doctors proposed an experimental therapy. That would allow her to live on in a sense, her daughter could receive a mental transplant of sorts. Not a true personality switch, but she would have all of her mother's memories, and possibly a degree of personality mixing. Two people as one, more the daughter than not, but also becoming her mother to a degree. Some of the more religious/spiritual doctors believing she would also at least partially receive her mother's soul. To truly live on in a sense, perhaps even aware of everything from the corner of her daughter's mind.

You did not hesitate to go through the procedure. Unfortunately your mother died in the process, but when you woke up, you could really remember her whole life. Perhaps you did even feel something you wanted to believe was her soul entering your body.

Now we are back home, but knowing what happened, can the relationship between father and daughter remain the same. Or will they be pulled into a different, secret relationship. One that they may or may not be able to keep secret.

That's the idea, a story heavy idea, but with plenty of sex mixed in and a somewhat more natural evolution into an incestuous relationship. A relationship confused by the union of mother and daughter. It does not immediately turn incestuous, but both find themselves increasingly drawn to each other, as man and woman.