I had two children. I want you to be as free to not have children as I was to have them.

The EvoDevo types will rag on you as denying your true being, either siring or birthing kids, but it's horseshit. People who don't have kids are often making it easier for the rest of us by taking up demanding professions, and yer helpin' the herd ANYWAY just by existing, whether or not you have bebbies.

1. One of the biggest differences between animals and human beings is that we get a choice about being parents. Refusing to have kids has many causes, but let's set "because I can differentiate myself from an unthinking animal that way" close to the front of the line.
2. If you've decided that you are unfit to parent, what's weird is that you've acknowledged it, not that you've decided not to do it.
3. If you have learned or were born with a genetic defect that you choose, freely and advisedly, not to pass on or ask a partner to help raise, then people should feel compassion, not give you drama.
4. One of the biggest mistakes people make - or accidents that happen - in life is having children. People who regret it can be quite trashily vocal about how everybody should enjoy it. Back away while making civilized noises when confronted with these assclowns.
5. Some people have been traumatized in their childhoods, and having kids would be one, long, fucking, horrific flashback. Strangely, in some families the people yelling at you to have kids are the same people who gave you every reason in the world not to.
6. Selfish? I don't even - I just don't - how can not having children be selfish? Is it the carrying on the name, is it the make me a grandmother, is it the you'll be too focused on yourself? If the people howling at your lack of progeny cared about your life rather than their asinine, overstuffed social conventions, they'd shut up. Weird? No. When one person in five is doing something, it's not weird. Different? Only in that your reason for remaining childless is uniquely yours, however much it looks like someone else's.
7. How the hell do they treat people who are infertile? Just asking.
8. I won't even touch how not having children can be considered ethically superior, which kicks the whole selfish canard in its hypergametic nads.

Hope you find the right things to say and do to keep things cool in the family.