I think it doesn't matter what choices we make as parents, there will be always somebody to prove your wrong and criticize you. If you were formula feeding there would be somebody to tell you off that you should breastfeed. And so on. In my opinion, you should not try to defend yourself and your choices, be firm and don't allow debates regarding your parenting decisions. You won't change their minds but if you say firmly that you don't want to discuss it and change the subject you may have a more pleasant time with your family than if you were defending your position. Besides, you have nothing to defend, you should be proud of yourself
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Typical exchange would be like this:
Relative: "You should give this baby some formula, it would be better for both of you."
You: "We are happy with our decision to breastfeed, do you want some more tea or a cookie?"
Relative: "But blah blah blah..."
You: "I don't want to discuss this, let's talk about something else. Have you seen the last episode of..."