It would depend on my sister's age. If my sister was really young, I might be worried about the influence of this friend on my sister. I would also consider my sister's maturity in handling difficult situations. If my sister were 25 and the friend was about this age, I would be less concerned about the situation than if my sister were 14 years old and immature.
So it all depends on the circumstances, the ages of the friend and my sister, and my sister's maturity.