You know, I've been a Scooby-Doo fan since I was a kid, and this question actually sent me down a rabbit hole of Hanna-Barbera lore! Scooby-Doo himself doesn't have any canonical sisters in the original series or most spin-offs. The closest relatives we ever see are his nephew Scrappy-Doo (who's... divisive among fans) and his cousin Scooby-Dum, that goofy, slow-talking Great Dane from 'The Scooby-Doo Show'.
Interestingly, Scooby's family tree gets weirder in prequels like 'Scooby-Doo! Mystery Incorporated'—there's a whole ancestral line of talking dogs, including his great-grandfather. But sisters? Nope. If you're thinking of female Great Danes in the franchise, it's probably Daphne (human) or maybe Dynomutt's sidekick, though she's not related. Makes you wonder why they never gave him a sister—could've been fun to see a lady-Doo solving mysteries!