Rename Bill's brother to Bill's older sister

[metallicity]

I noticed that NPC attached to BillsBrotherScript had a sprite of SPRITE_POKEFAN_F, and wondered if the labels here were maybe incorrect. While I couldn't find anything beyond the sprite itself that seemed particularly definitive, the same house in HGSS also used a similar sprite, and Bulbapedia currently describes Bill as having two sisters (rather than a sister and a brother), so I went ahead and wrote up a commit for all the related renames.

Bill having two sisters does introduce some potential for naming collisions (i.e., BillsSisterScript), so I used BillsYoungerSister and BillsOlderSister to avoid this. The Goldenrod NPC being younger than Bill is perhaps a bit more clear from the in-game text/sprites, but Brent's phone gossip script about "Bill's younger sister" would technically be ambiguous if both were younger than Bill, and regardless the Fuchsia NPC would be the older of Bill's two sisters.

[vulcandth]

Just wanted to leave a comment, that we haven't forgotten about this, but have been a bit busy with some life and other projects. I'll need to review your references and look over everything; hopefully have an update for you this weekend. Thank you for the contribution regardless 😄