i'd say you are making a good start anyway... .why not block the sister and nephew as well... .if you don't you could possibly get unwanted status reports through them as well... .and any mutual or non mutual acquaintances... .just sayin... .if you want complete nc... .cover ALL the bases... .
I appreciate what you are saying and I think it makes sense for almost every situation.
It looks like blocking her knocks every possible status update from her that she might post on the sister or nephew's page. I am friends with the nephew and sister has always been nice. They are in Mexico and if I blocked them they would consider it rude and probably disrespectful. OTOH if the exgf tells them I blocked her the sister and nephew might block me.
I have been through this before with them and they blocked me in 2011 or 2012 (I forget) after a nasty breakup with the sister where I wasn't particularly nice.
The exgf will be in warp speed to get a replacement and try to rub it in my face. i don't need to know.