The first thing to bear in mind is that your breastfeeding relationship is a private one between yourself and your child. Although everyone is entitled to their opinion, you need not feel that you must defend or explain your choices.
Having said that, it is also important to recognize that people who sound critical may simply be inexperienced with this topic. After all, most tandem nursing mothers simply “find” themselves there, it’s not usually something that mothers set out to do from the beginning.
If you wish to offer a little education, you can let the other person know that it is safe (and point them to the latest information) and that breastfeeding offers nutritional and immunological advantages for the entire time a child nurses. Let them know that as the mother, you are assuming the responsibility to make decisions around your children’s health, and that after looking into it you judge this to be for the best. If the person worries that it will take a toll on you, let them know the ways in which continued breastfeeding is helping you, for instance in finding time to put your feet up while meeting the needs of baby and toddler. (You can also suggest a way for them to genuinely help you conserve your energy, by making lunch for your toddler or taking him to the park!) And when it comes to tandem nursing it is always fun to remind people that we have two breasts, and that yes, miraculously, a mother’s body can make plenty of milk for all.
See also: Handling criticism about breastfeeding