I would only do it if invited to - either if the converstation got round to the topic or if you were directly asked your opinion on it. Because if you look at if from your own point of view, how would you have felt if you had someone get in your face about the options you had - obviously you are for a very low/no intervention birth and that is awesome, but not everyone is and I imagine that if someone bombarded you with stats about birth, especially if those stats were 'bad' ones about the option you chose, then you would get pretty tetchy, so you have to reverse the role and respect that not everyone is going to be open for discussion on this.

if you really want to be proactive and not wait for that 'invitation' then maybe you could suggest a reading list of books that you found really helpful etc and leave it at that for the time being and maybe after a while ask did they read them and what they thought, because gauging their response to that will probably give you the opening you want to talk about it further and in more detail. If you don't think they were receptive to the books you recommended, then you will possibly be flogging a dead horse if you keep going on about natural birth if they feel that the whole shebang would be better for them kwim?

It really is admirable that you want to make sure that they are informed, but birth is such a personal choice and at the end of the day we have to respect that those that we are closest too may not always see our point of view. Good luck