A few people have already mentioned this, but in case it's still not quite clear. Dylan isn't setting up Cedric with his future GF for the lulz or because she ships it, but rather because she's concerned that the novel's plot might act as a kind of predestined fate if she doesn't. Thus, her biggest attempt at breaking fate is actually setting up Cedric with what's-her-face prior to getting pregnant with Cedric's child so that the fate of "Cedric's first wife" is avoided altogether.
In short, she's convinced that unless she can make that relationship happen, her free will be in some way limited by the novel's plot. Becoming the First Wife character instead of her sister, after all, is a minor change (especially if Emily later turns into the novel's Dylan-esque antagonist) compared to just bypassing the First Wife tragedy altogether.