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Sambastian Romance is a mod makes Sam and Sebastian a couple! You can download it here: https://www.nexusmods.com/stardewvalley/mods/10483
It adds/edits dialogue from the two of them and others to reflect their relationship. Cutscenes up to 8 hearts are edited to be platonic with the player character and reference their relationship. All references to romantic relationships with other NPCs, i.e. Abigail/Sebastian, are removed.
Both of them remain romanceable, but given that the one you don't romance will still act like he's in a relationship, I suggest you just uninstall the mod if you choose to romance either of them.
On rainy days, there's a 50% chance to see a surprise! Follow Sebastian around and see what happens ;)
Installation Requirements: SMAPI and Content Patcher. Follow the standard installation procedure for CP.
This mod has not been tested on Mac/Linux.
Screenshots
Let me know if you try it out, especially if you find any bugs! I hope you enjoy!