A man dies in the present, is sent to the past, Fairly standard, so far. When he arrives in his new body he is one half of a set of fraternal twins, and now female. Again, been done, with the gender switch at least. His(her?) twin is a geek, and she(he?) is gorgeous but has no desire to change their orientation. Since it is not the 90s yet, orientation is a problem. The solution? Create a Harem for her brother, with the initiation being a threesome with the two of them. How she makes that happen would be part of the build-up.