They are both so pretty! I would describe lovely Luna as a tortie Domestic Longhair, and sweet Starla as a dilute tortie Domestic Shorthair. So, they aren’t mixes, but, rather, the landrace cat breed. If you’re curious, you could get them DNA tested, which will tell you if they have genetic similarities to any breeds.
No they’re not siblings. I have a personal love for Tortie cats. I rescued both of them from the animal shelter at different times. I was attracted to Starla’s Tortie cost but didn’t realize there’s a name for hers “dilute Tortie” until one of you mentioned it!