Article continues below advertisementKourtney Kardashian celebrated Kim Kardashian's 45th birthday with a heartfelt throwback post that had fans swooning.Article continues below advertisementThe Lemme founder shared a carousel of nostalgic photos on Instagram."Best friends forever," Kourtney wrote in the caption.The first snap showed the two striking a pose back-to-back in matching bob cuts, giving off major '90s vibes. Another picture featured the sisters in cozy robes, their faces covered in what looked like a homemade face mask.