The letter shows all the manipulation you would expect. You have to make changes, they are feeling rejected by you and they are being forced to exclude you from one of the most important days of their lives.
Despite the protestations of love it comes across as very cold. If you feel the need to respond than make it clear that you have not rejected them as they suggest but that their decision not to associate with you is theirs alone. E.G.
We all love you very much and cherish the importance of family in our lives. As a family we share experience and upbringing and have a unique bond that is not easily broken or lightly cast aside. This means that we still love and respect you and appreciate all you have done for us. Our love for you is not conditional on what you believe, who you associate with or how you chose to live your life we are always available to you. We will not reject you or exclude you because you are that important to us.
You are right that we all have choices and we choose to love you always just as we love all our family.
However bear in mind that any response is likely to fall on deaf ears they will not be open to reason. With this in mind sometimes it is better to ignore the manipulation rather than fuel the fanaticism.