@knorvy10 It's just a double standard - his baby sister had no concept of privacy, nor should she. There's nothing shameful about being naked, much less for a baby but really for anyone, except that some patriarchal cultural systems tell women to hide their bodies.
That said, I wouldn't be too hard on him because the sexualization of females (regardless of age) and the commodification of the female body are likely entrenched in his psyche from years of conditioning, and that means he very likely
does think of every vuvla as sexual and/or shameful, and may in fact think girls (and women) have a vested interest in concealing themselves (for modesty and to keep themselves safe). He may not realize that view is unnatural, and just an offshoot of social constructs around gender identity and sexuality.
The bad news is that changing that framework takes volition and effort over time, so unless he acknowledges the problem and starts to work on realigning his views you will likely have to navigate around it. If he does take a pause and consider the destructive consequences of his views, it could take years to adjust them but it will be worth it imo.