Everything we do with regard to other people is an attempt at external validation, including this post I'm writing now, everybody's FB profile, all the posts on MN, every time we meet a friend for a chat, family relationships, having sex, everything. We are societal creatures, and we all need to feel a part of something, and to feel accepted by others.
The problems come when people differ in the level of their need for this. Many find seeking external validation via posting about a funeral distasteful, but many don't. Many find posting pictures of your dinner a bit too much, but many don't.
OP and her sister have different levels of acceptance of this level of validation need. There is nothing that can be done about that. Sometimes the behaviour of somebody we love can seem unsuitable. That's just life.
There is a line though, with regard to posting a picture of somebody else, who doesn't want it posted. The line is there because there are rules about that sort of thing. Which is why appealing to a higher authority is an option here, if the individual insists on disrespect.