I have not read any comments so far so I will be going on my own instinct of what you wanted to happen at that event. A good while back, I was told I had the best camera (
) so I should be the photographer at a family event (my husband's). It was casual event too. I made it a point to stay very far away from the crowd and set up my tripod shooting expressions, playing children, misbehaving teens and some busy people preparing the food and was certainly had a good time without anyone really asking me for a shoot. I came out well in the end because I just shot what I wanted to shoot, nothing in particular of any theme...In short, it helps to go there with nothing in particular to expect but because this is your mother's big day and you are located in a Commonwealth territory/country like Australia, I will suggest that you take a picture too of the Queen's letter to your Mom congratulating her for reaching the age of 100. If this is going to be framed, then frame the letter alongside your Mom's image. I am sure she will be thrilled.
Just a thought...