That implies someone had given birth to them. ( No everone has niblings.)
And who said that they're parents would allow you to teach em your world view? ( Especially as those parents did create children and you didn't. Hence I presume they would also like to have grandchildren. And you would discourage that.)
True, but also friends and family are usually either same age or older. ( as in the other response) I assume parents would prefer to have grandchildren, so they would not apprecheate your anti children sentiment spread by you to they're children.
True, adoption is also a good thing generally. But to help the genetical pool be diverse and healthy some children of your own wouldnt be bad.