Thing is when I had my DC1 she was single and used to visit us loads. Dotted on our DC1 bought her lovely presents. That’s when we got on really well. I am two years older than her I had DC1 at 37 and my DC2 at 42. She had her DC1 at 41 and DC2 at 44.
It sounds like she was relaxed about you having DC when she was 35 and presumably still optimistic about meeting someone and having her own DC but started to resent you after she had found a partner, been "not not trying" for a few months and had become afraid that it wouldn't happen as easily for her as it did for you.
It is very unfair on you as you didn't cause her difficulties. However you sound quite dismissive of the trauma she went through, having multiple miscarriages when TTC in her forties must have been really devastating for her. Yes she had children in the end but at the time she had no way of knowing that it would work out that way. You don't have to look far on here to find threads from women struggling with miscarriages and fertility issues saying how it can be really hard to maintain close relationships with friends and family while fighting jealousy over how they may have conceived easily.
It does sound like you may have made things worse with the joking about TTC and commenting on her breastfeeding. Even if you didn't realise it at the time, those are really sensitive topics and she probably felt very vulnerable and insecure. And your DH was also not helpful, when someone is struggling and grieving a miscarriage is really NOT the time to have it out over other relationships.
You also mentioned that your FIL dies recently and that she was at a funeral when she snapped at your DC. If that was her father's funeral or another funeral soon after his it's hardly surprising she would be emotional and not at her best. And you don't at any point in your posts acknowledge or express sympathy for her having lost her father.
It doesn't excuse her being unkind to your DCs but I think it might be helpful if you could accept that at least some of her feelings are understandable and due a really traumatic period that you (unknowingly and not deliberately) weren't very sensitive to. I see that you have tried to offer practical support and maybe you have given more emotional support than comes across in your posts but something to think about maybe.