𝐒𝐢𝐬𝐭𝐞𝐫𝐬 𝐒𝐮𝐦𝐦𝐞𝐫𝐬 𝐢𝐧 𝐬𝐡𝐚𝐝𝐨𝐰𝐡𝐮𝐧𝐭𝐞𝐫𝐬 𝐰𝐨𝐫𝐥𝐝

In which two sisters - Buffy and Dawn Summers, after the recent events in Sunnydale, move to New York to start
new life and accidentally enter the world of the Shadowhunters and to fight the coming evil.

or

Where Alec Lightwood falls in love with a vampire slayer

Alec Lightwood x Buffy Summers

Simon Lewis x Dawn Summers

the action takes place after the 7th season of Buffy the Vampire Slayer and season 1 in Shadowhunters.