My sister took me to court after mom died. When she did that, she cut the bloodline, including cousins with my kids. She wanted to 'friend' me on Facebook, then she started following me.
I declined friendship, and blocked her and she can't follow me anymore. Privacy Setting will give you a list, who you want banned, or blocked. There's also a HELP, section, to answer all your questions about who you can filter or where you want your post (friends only) or just me. Your choice. But know this, if your sister are friends with YOUR friends, when they click 'like' she still can see what you posted.