It is kind of complicated, whichever way you want to decide the "big sister" role. Mentally, Kuro is older by what was it, 8 months? Physically, Illya is the one in the original body, so she's arguably older in that way with Kuro only having been in a body of her own now for weeks? A month or two? It's kind of like all those reincarnation type of stories, how you want to count the age can be fairly subjective.