Way, way, way too small. No offense, but since she hasn't done her research on the deadly risks to her female, I doubt she's done the neccessary genetic testing and all the other research that is so vital to a healthy litter. Your sister is not ready to breed period, even if her female was larger. That tiny girl should be spayed to prevent any accidental litters that would probably kill her.
Good luck convincing your sister! Its nice that you care so much!
