Sofia, sorry, but you are nuts.
A nanny is an employee. At work, I can't remember getting miffed at not being invited to the Chairman's wedding when he got married. Where on earth does being a professional who competently does a valued job entitle someone to an invite to a wedding? Nowhere.
I absolutely adore my nanny and have said so on these pages many times. She is brilliant with my children. But we have a professional relationship, it does not mean we are lifelong buddies. The people I got married with are the people I hope to still be great friends with in 30, 40 years' time. Loving somebody's ability to care brilliantly with my kids does not mean that they are automatically going to be a great friend of mine. And to be honest, my current nanny would be bored rigid spending a day with my friends.
Hollyn, yes, I am sure it would have been lovely to have been invited to the wedding, but that's not the relationship you have with your host, and you are there to work. So long as you were given the choice of doing this work or not, then I don't think you should get uptight about it(and FWIW I strongly believe that the shared bathroom thing entitles you to have said No if you didn't want to go - assuming of course this shared bathroom thing is for the whole week that you are talking about?). Last Saturday I went to a ball with my husband at a place where he played rugby before I knew him. I didn't know a soul there, had nothing in common with the wives and girlfriends, and spent much of the evening being bored to tears by the wife of the coach (who at one stage gave me a blow by blow account of the transferring of koi carp from one garden pond to another). It is really shit going to events like this where you are not 'one of the gang'.
And of course, the close family will have taken all the best rooms. That's what happens at weddings. And it is nothing to do with you. You would do the same if it was your wedding, especially when people are spending shedloads of money coming from all over the place to be at the wedding.
I would try to ignore any feeling of being slighted, and treat it as just part of work, nothing more, nothing less. Not being invited does not mean they do not hugely value you and your role. Maybe the mum has been a bit insensitive in not realising that you are feeling slighted, and she should maybe have had a chat with you about it all, but in the heat of wedding madness there are a thousand things that slip through the net.