Blocking will just lead to a rift/going NC.
You can go LC without blocking. Although arguably you are that already at 3x a year face-to-face contact.
I am sorry you feel alone and let down.
It is difficult especially when everyone tells you it takes a village to raise a child - and when that much longed-for child - is more challenging to cope with. That in itself is very isolating.
However, you cannot make people support you. Many on here, myself included, have learnt that the hard way: so many have Disney dads/reckless ex husbands or mums who do not want or are bothered about seeing grandchildren or both.
If, as a single mum, you have no partner or parents and relied on your sister, who has not stepped up as she has her own caring responsibilities and professional duties, that is understandably sad for you but the resentment/bitterness will not help long term. You are going to have to build your own village or accept the buck stops with you. There is little respite for any mums with children with SEND. But there are groups and services out there for you to find in your local area.
I take mine to community swimming each week to reregulate. I do low-demand parenting much of the time apart from hard boundaries of teeth brushing, sleep and school attendance (latter not perfect but better than many). I am on here a lot.
Try to be upbeat rest of time as noone likes to be around misery, moaning or neediness (even though I am all three). Work helps me as well as hinders, as it keeps us to a routine and gives me adult conversation I would not get otherwise.
Like you, I have no family to help out.
Unlike you, I have a term-time only job on purpose.
The stress of working in education nowadays cannot be measured unless you are in it so I am on your sister's side on that one. Does not mean she could not offer some respite, a day or two, hosting you or having her niece BUT with four of her own on top? I'd be struggling.
Plus, as any teacher will tell you, as soon as the adrenaline that has kept you going all term stops, you often slump yourself in the 'holidays' and keel over. So I cannot blame her for putting on her own oxygen mask first.
Good luck to you 🍀 Unblock her love. It is cutting off your own nose to spite your face otherwise.