Any stories where the male lead and female lead like each other, but the male lead's brother causes problems for them because he's also in love with the female lead.

Faster Than A Kiss

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Anitomo Not brother.. but cousin? Still part of the family? XD

goong

Hobbies include impersonating fictional characters and sucking on ketchup packets.

Watashi ga Motete Dousu n da has this as a minor plot point later on, but might not be enough since it's not the entire plot of the series.

I think Akuma de Sourou

There's a category for "Younger Brother" you could always search which might fit or even "Brother/s" or something.

Watashi no xx Shinasai! brother tries and almost accomplishes to steal away girl major part of the story too.

Meru Puri older prince brother tries to steal girl.

Beauty Research Club beautiful-ish people... Kind of I mean everyone has their own tastes...

The Sleepy Residents of Birdcage Manor they're cousins don't know if its okay? And the story is good too.