i think having an audience of 300+ demands a new set of social rules.
Never mind the fact that what are the social rules in one area are not necessarily the social rules in another.
My sister posts a bunch of mumbo-jumbo ALL.THE.TIME that isn't in line with my and DF's belief system. Her friends will often comment how wonderful those posts are. I, OTOH, think they're a bunch of poop and have learned since to hide her posts (it's worked out great so far). Does my sister know my belief system? Yes. Does it bother me some of what I used to see in her posts? Yes. Do I think it's rude of her to post something that bothers/offends me? No. I think, "Her wall, her rules." However, I don't have to read her posts...so I don't. Interestingly, SHE has tried to dictate to ME what I can post on my own wall and ignored me when I've requested she take her controversial comments to a PM so I've deleted her posts in the past (I've tried forewarning her what's going to happen but then she'd post on my wall anyway, instead of reply to my PM). She no longer tries to control MY wall and will occasionally "Like" something on my page.
Really, I'm amazed at how at odds everyone is about this. Even the laws of the land haven't kept up with technology. Do people really expect everything else to have? I think this is definitely a case of YMMV. The OP is bothered by her niece's posts and can opt not to see them (if it means missing all other posts by the niece, then that is for the OP to weigh what is more important -- hiding the wedding posts or being able to read everything). I just don't see that the niece is rude by posting about her own life on her wall.