uhhhh...I’d venture to say NTA, but this whole situation is setting off red flags for me...it’s obviously super weird that they’re so ‘close’, especially assuming they’re biological siblings? Even if they’re step siblings, this type of shit should be a no no. I guess maybe they were raised by parents who taught them that nudity isn’t bad or something they should care about since they’re family, but if this is as weird for you as it would be for me, I think it’s time to tell your GF exactly how you feel. If she isn’t okay with like not being naked around her 17 year old brother, maybe this isn’t the right relationship for you.