Personally, I would either follow one of two options. 1) completely ignore the email and pretend you never read it. Continuing to behave in regards to breastfeeding as per the normal.
2) write a response along the lines of ' I appreciate the awkward position you may feel you are in, between a mothers legal right to breastfeed and your apparantly not so open minded relatives. I am giving you the benefit of the doubt in assuming that someone 'must' have already said something for you to dare hurt me in the manner in which you have. Breastfeeding is not 'optional' for me. It is part and parcel if the package. You either trust me to feed as i need to feed in a manner to which my baby us accustomed, or you do not. You cannot impose restrictions on how i may or may not do so, unless you are imposing restrictions on all your guests as to how they might breath and eat or talk. I would not wish to upset your day, I will humbly allow you the decision of my attendance, if you feel you cannot accept mine and my baby's prescience then that is your decision."