If someone sends nudes to your big sister, she should immediately block the sender. And she can consider reporting the incident to the relevant authorities or platforms, depending on how it was sent.
Of course. On Tumblr, for example, you can adjust your privacy settings to block specific users from seeing your posts. You can also set your blog to be private so that only approved followers can view your posts, which is another way of blocking unwanted viewers. It gives you the power to curate your audience and share your content only with those you trust or want to reach.
There could be several reasons. They might be shy or not want to show too much interaction. Or they could think the stories are okay but not good enough to warrant a like. Maybe they're just passive viewers.