Considering that she is making her life public by writing a blog she is open to others interpretations/comments, etc. Since she directed someone to her posts about how to do things without insurance I think it is a fair assumption that they do not have insurance. It is fine to have a large family if you can afford it but not so much when you live in poverty and have no $ to buy food, etc. No one is doubting that she is taking good care of her kids but being a responsible parent goes further then that. It is also providing healthcare, some activities, worthwhile experiences, etc without spoiling kids. Not to be so dirt poor that they only survive. The fact that they have no $ to buy food but must live on the pantry, etc and then still continue to have kids-sorry but I & others think it is irresponsible!
I agree that you have a right to fair comment. Fair comment is limited to truth and opinion. Opinion on facts is fine, making up facts that are not based on truth is not. Making up facts that would bring someone's reputation into disrepute and posting them online is libel.
Let's examine your statement: " (a responsible parent is) also providing healthcare, some activities, worthwhile experiences, etc without spoiling kids. Not to be so dirt poor that they only survive."
1. We don't know about healthcare - but it is available for all the kids at a cost of $100 a year at their income level. Given that we do not know, you can post your opinion that it is necessary and that if she does not have it you disagree with this choice, but you cannot state that she is irresponsible for not having it when you do not know it to be true without being at risk of engaging in defamation.
2. The kids are involved in archery and many church and extended family activities. They have music and sewing lessons. They do a large amount of craft and science projects. They have an in ground trampoline and playground equipment in their yard including a merry-go-round. They do daily chores together. They have and attend birthday parties and parties on other occasions. They volunteer a lot of their time at the church.
3. Worthwhile activities - see 2 above. I personally know a lot of kids who spend time on video games/computer - my kids included. I do spend a lot of time with my kids because I work from home and they have music lessons and play sports and have lots of friends, but my impression is that Brandy is doing a better job on the "worthwhile activities" aspect than I am. She has devoted her time to her family, has schedules for schooling, and this is pretty valuable in my books.
4. "Not to be so dirt poor that they only survive"... the sense I get from her blog is that she has a deep sense of satisfaction with her home, garden and family life and her children seem very happy. It is far from "merely surviving". When I finished law school I had to work long hours. I made money, but it was "merely surviving" because money is simply not the measure of a good quality of life in my books. I soon quit because I was merely surviving.
Where I would agree with you is that a little extra money would go a long way in their situation. I would look for a small pt job from home - which she has in her blog. This is merely my opinion though.
I also agree that public figures are subject to a slightly different standard re. defamation in the US. However, having a blog does not necessarily mean you are a "public figure" and even "limited purpose public figures" and "public figures" can sue for defamation when untrue statements are made that are calculated to bring them into disrepute.
You may wish to reference this US guide for bloggers:
https://www.eff.org/issues/bloggers/legal/liability/defamation#7