My sister thinks it's okay to be topless and drinking a bottle of wine with a straw at a family barbecue or normal beach (non-nudist that is)

Of course it's ok why wouldn't it be ok. Being nude or naked with others be it family or friends and even strangers isn't a big deal or sexual to be nude or naked, its a perfectly normal and natural thing be nude. We all know what naked and nude people look like and have seen it all before. No one cares or takes any notice.