OP, I don't think it's about what went down regarding your son and your brother's wedding. I highly doubt that your sister would take such a stand about that, given that it wasn't her own wedding but your brother's. I feel that it's almost certainly about her thinking you weren't there enough for your mum and now your dad. She lives there and so did much, much more, and was there for all the horror of it all. I have been in her shoes, the sister living with a sick, elderly parent. There are truly terrible moments, like in my case when my dad collapsed in the shower and another time when he had a bad reaction to some meds and went temporarily loopy. He thought I was trying to poison him. It is truly awful to be on your own with the patient at times like that. Even though she wasn't on her own when your mum was ill, as your dad was there, she might still resent that you didn't have to live with it all.
I know that you have your challenges with your SEN children, and so I don't share your sister's possible viewpoint that you got off lightly. I actually think once a week visit from you is pretty generous. My sis rarely came, and when she did, we were treated to 36 hours and that was it. (2.6 - 3.5 hour drive for her, depending on traffic.) But from your sister's point of view, she had to be there for your mum's sickness and you didn't. She may also not realise how hard it is to care for SEN children or what your challenges are.
She might wonder why you couldn't let your DH look after the children for 3-4 days sometimes so that you could have stayed with your folks. And given her a break. I cannot describe how hard it is looking after a sick parent with no break.
Anyway, I have been your sister. Like her, I was free to live with Dad when sick and take care of him, as I'd split up with my husband too. It is hard to be the sister under the constant strain and witnessing all the suffering and having lost your marriage, while your sibling, with her intact marriage, waltzes in and out for a handful of hours occasionally and then back to her home and husband.
This is why, fair or not, I think it's almost certainly to do with unevenly split caregiving duties.
But it's very difficult when someone won't talk to you and won't tell you why. Maybe she just needs time to simmer down and move past it. I think you should leave her be for the foreseeable future, except obviously still get her a card/present etc at birthday and Christmas, and check in with her sometimes. But I would give her some space. And I wouldn't get the special gift this birthday. She'll see it as pressure.