Desperate You have written a detailed and thoughtful post, clearly outlining the whole problem and acknowledging your own and others culpability within the situation. From what you describe you are right to be concerned, on so many levels.
You are loyal to your wife but I do think that from what you say you have been allowing/enabling her behaviour for a very long time. I imagine that over the years you have been aware of other behaviours that have been of concern to you and are now being seen in your daughter as well. I think you really do have to "bit the bullet" now, you cannot allow this to go on, however hard it might be and also however scary!
I am not sure that showing her this thread is a good idea; it seems to me that would be a recipe for making your wife react angrily, defensively, go off on a red herring of your "disloyalty", deflecting all of her responsibility for the situation away on to "your faults"!
Your first priority must be to stop the planned contact with Social Services, Health Visitor and Doctor. Tell her that this is completely wrong and unjustified, and that you will not support her in this at all, you will be telling them that you feel your DIL is a perfectly good mother, that there is no evidence of what your wife describes. Tell your wife that you will simply not condone her planned actions.
If she does go ahead with contacting them, you should stick to what you have said, contact anyone she has contacted and tell them quite clearly that you think your wife is wrong. And tell your son and DIL that you have done that. Tell your wife you have as well.
Then you need to start addressing your wife's wider behaviours with her. Use all the points that you made in your original post, your own culpability, your lack of speaking up about your concerns etc. Talk to her about it just is not her business the choices that your son and DIL make regarding breastfeeding and equipment. Try to get her to be honest about her own anger that she could not breastfeed but that it has nothing to do with what her DIL does. Your wife really does need to be helped to see what she will lose if she does not change her behaviour.
I suggest you also speak to your son, tell him that you understand that his mother's (and sisters) behaviour is inappropriate; tell him that you are speaking to his mother and that you will support him as he steps up and supports his wife. Tell him that is absolutely his priority NOT his mothers feelings on this one!!
You say that you love your wife and that she has many good qualities. Inevitably within the parameters of trying to explain your dilemma in your post, it is difficult to know what are your wife's strengths and good qualities but if you are clear what they are then use those as part of your discussions with her to hopefully help her to listen to all the difficult messages that you have to give her.
You know that you should have said something a long time ago. The fact that you didn't does NOT make a bad father and husband as some have suggested, sometimes it takes a long time to admit/see the truth of a situation. However it does make it imperative that having seen what is going on and the impact on your DIL and your son, you do something NOW as a matter of urgency.
Do please come back and let us know how you get on and ask for help and advice if you need it as things develop 