Your mum is trying to manipulate you in order to gain access to your father's wedding plans. Her motives are self serving and have nothing to do with chaperoning you. You are right, there will be plenty of people there who can watch over you...not that you even need watching over as you are a teenager, not a toddler. IF your mother was welcomed to attend her ex-husbands wedding, then she would have her own invitation.
Tell your mother that your father has made provisions for you to have a traveling companion along with someone your age to spend time with at his house and during the wedding festivities. It is crazy for your mother to inject herself into her ex-husband's wedding plans and to use you to do so.
Whatever happens, DO NOT stray from the plans you and your father made. You are in the right here and how dare your mother try to manipulate and twist things in an attempt to bulldoze her way into her ex-husband's wedding plans. Shame on her.
EDIT: Not to scare you, but young girls traveling alone are targets for criminal activity. So, you be sure to NOT speak to strangers, even friendly ones. Always be aware of your surroundings and give no information to anyone about what you are doing and where you are going. Stay close to other adults so you are not isolated, giving bad people access to you. Keep in cell phone contact with an adult who knows where you are at all times while you are traveling. This is for your own safety.