Brother’s relationship

These are historicals with brothers as an important part of the plot. Most of these do have romance, though:

• The Untamed (censored BL that can be seen as bromance if you prefer, both of the MLs have brothers and their relationships with them are a huge part of the drama)
• Ashes of Love (MLs are brothers but this is definitely a romance and there's a heavy love triangle between them and the FL. However the brothers do reconcile in the end)
• Arthdal Chronicles (minor romance)
• Lost Track of Time (love triangle between brothers and the FL, MLs have a very complicated relationship and one kills the other in their former life)
• Love Between Fairy and Devil 
• Moon Lovers: Scarlet Heart Ryeo 
• Tale of the Nine-Tailed/Tale of the Nine-Tailed 1938 (the second one much more so and has only a minor romance)
• The Secret Romantic Guesthouse (two of the MLs are later revealed to be half brothers)

Also these aren't historical but focus a lot on brothers:

• Bokura no Shokutaku 
• Duel (no romance, and the brothers are clones with a complicated relationship.)
• At a Distance, Spring Is Green (ML has an older brother and their relationship is an important part of the second half of the drama)
• Come and Hug Me 
• La Pluie 
• Not Me 

This list is what you are looking for:)

https://mydramalist.com/list/p1RzPQ81