OP,
I am heading for 60 and have a few friends that did some community nursing over the years, as trained mid wives, and breastfeeding specialists.
They have told me very clearly that while some women do get PND of the full mental health breakdown type, a huge number get a version which is hugely connected to their relationship, their partners support, general support and family dynamics.
Iron levels were always the first thing they encouraged to be checked as low iron and the general tiredness can also contribute to feelings of PPD.
It is IMO completely believable that a young mum breastfeeding and exhausted, having a stressful hostile environment around her, and seeing her partner tolerate it, would trigger huge stress and PPD.
I have no doubt most women would.
I can well imagine you holding onto your anger too.
Purely for YOU and your well being, would you consider some counselling and maybe some joint counselling so you feel very heard and your husband gets to hear just how big a fxxk up he made?
I know you no longer see much of them, but he knew how difficult she was and yet he did not protect you when you were so vulnerable.
This is the key I would think to your anger.
At your most vulnerable, having given birth to his child, knowing how his mother is, he didn't have your back.
You have suffered a profound lack of trust in him, as a man, husband and a father.
His actions have damaged your relationship profoundly and you need help to move forward if you can.
Over the years friends of mine have had issues with in laws, but where the husband did not support them sufficiently, was where the real relationship damage was done.
You need support to move forward.
On a positive note, your family sound lovely.
Wishing you the very best.