How does one establish a "derived work" relationship between this post and post #2255083?

If it's by the same artist, like in this case, just parent them.

derivative_work is for when one artist is inspired by (or models his artwork after) another artist('s).

Great, thanks!

hungkok2007 said:

Great, thanks!

And since this is the pixiv upload, make this one the parent :3.
EDIT:
Even if the size of the parent post is much bigger?

The pixiv > twitter parenting isn't an inviolable rule, it's more for when you the same resolution picture has been uploaded to both. If the twitter source is much larger (or noticeably higher quality), there should be no problem making it the parent.

Gollgagh said:

The pixiv > twitter parenting isn't an inviolable rule, it's more for when you the same resolution picture has been uploaded to both. If the twitter source is much larger (or noticeably higher quality), there should be no problem making it the parent.

But I thought twitter is doing recompression so that the upload isn't first-hand source.

The key point is if the twitter post is still higher quality than the pixiv, having it as the parent is fine, twitter jpg resampling notwithstanding.