We have the same problem, and it infuriates me. It should be no one’s business but ours, how many children we have and when.
I think that when family oversteps their bounds in this way, repeatedly, then the time to be nice has past. Next time a relative brings up the subject, tell them very firmly that you will live your life as you see fit, which is in accordance with the teachings of the Catholic Church, and that this topic is no longer up for discussion. Period. Then change the subject. Repeat as necessary. They should hopefully get the hint. After all, most of them would never tolerate you intruding on their personal lives in this way.
I hope this helps,