It would be, except for the part where when she not only posed for her own little brother naked, but he also showed that painting to everyone in his class. At this point, either treat sister's nudity as just normal art thing, or just come out as a hardcore siscon.
That problem, IMO, is not that.
The problem is asking the brother for permission. This is ridiculous, he should ask her directly, what the bothers feels and wants is irrelevant, she doesn't need his permission.