If your sister thinks Catholicism forbids wearing black nail polish, it is her responsibility to prove it. Ask her to provide you with the documentation that substantiates her assertion.
Whether or not your teen is allowed to wear black nail polish is something that is within your authority to decide. If you feel that it is simply an expression of her individuality and burgeoning independence, that would be a reason to allow it. If you feel that it is symptomatic of a bigger problem, such as an unhealthy interest in goth culture, that would be a reason not to allow it. Either way, your sister should respect your authority as the child’s mother and not inappropriately interfere with how you raise your child.
