is my girlfriend(F19) too close with her best friend?(F18)

me and my girlfriend (both 19) have been dating for 8 months. her and her best friend have known each other for 6 years, so i understand that they are very close. me and my best friend are close ourselves, but not to the extent they are. Her friend sleeps over at her house multiple times a week, she gets driven around by her quite literally EVERYWHERE bc she doesn't have her licence, she always steals her clothes, they go on vacation together, they work together, go to the gym together, not to mention they are way too touchy, they always hold hands or kiss on the cheek and my gf said they share the bed on sleepovers. And even when me and my gf are together SHES STILL THERE sometimes. its not that i hate her or anything, shes actually chill but i don't understand why they are LITERALLY inseparable. I love my girlfriend so much but this has been bothering me a lot recently, i feel like she puts her best friend above me. she argues that they just have a sisterly relationship but i can't imagine any siblings being that close. what makes me more paranoid is that her friend is bisexual and i'm scared she might have feelings for my gf. is this what female friendships are usually like? my gf says i'm overreacting but i don't think i am.