And it won’t, because they are two separate episodes. Plex matches against TheTVDB.com, which shows E15 & E16 as Family Reunion – and Farewell part 1 & part 2 (Season 4 Listing).
If you really want it to show up in Plex as one episode, just rename it to episode 15: Star Wars Rebels - s04e15.mkv.
See the Multiple Episodes in a Single File section of Naming and Organizing Your TV Show Files.
As mentioned in the support article, Plex does not handle this situation very well. You’re better off splitting the file into two individual episodes. See [HowTo]: splitting multi-episode files with MKVtoolnix GUI
