I think some people are unfairly ripping OP to shreds here.
I don’t think she has said her sister gets “too many breaks”- she has outlined that her sister has quite a lot of support with her daughter so this is not about a single mum trying to do it all alone, desperate for a break that she never gets. That’s quite a different thing. As it happens- assuming OP is being truthful- I actually think her sister is being unreasonable to push so much onto their mother, who is also working full time. She is also being unreasonable to demand that her daughter is treated exactly like all other children in the family- it’s simply not always possible.
OP has outlined that her niece gets daytime support from the LA during the working week during the school holidays, so doesn’t have childcare issues during holidays. She also explained that their mother (the child’s grandmother) has the child all weekend many (? Most or every) weeks, and also has her several nights per week. Their father makes sure he is available if she cannot attend school/care as she is sick and for hospital appointments. In addition, they have their granddaughter for at least one week per year to allow OP’s sister to go on holiday alone. I’m not saying this is too much, OP hasn’t said it’s “too much”, though she does worry about her mum as she is still working full time but also taking her granddaughter several nights per weeks AND at weekends. It sounds as though their mother does as much, if not more, care for the child as op’s sister. OP is (rightly) concerned about how fair that is. She is concerned because she has been told that her sister was angry and unkind to their parents when they decided to go on holiday themselves, which has made her mother too apprehensive to take another break herself. That is unfair. It also means that they seldom get time with their other grandchildren, which is also really unfair to both the grandparents and the other grandchildren.
Then you take in the holiday to the cottage. OP has outlined that as her niece has got older she has become bigger and stronger. OP is no longer able to physically restrain her if she runs off (something she does) or pick her up to remove her from a dangerous situation. She finds it stressful having to look after her, she requires very watchful attention. She feels that she would not be able to keep her safe if they went away and her niece started trying to run or throw things. She also just wants a nice break with her family, without the stress of it all. Her DH also does not want the stress and would like to spend time with his wife and children on holiday. None of this is unreasonable, especially when it is taken into account that them not taking their niece will not mean that her sister will have to forego her only break this year (when I’d say perhaps how they could work together to get get a break would be the nicer thing to do).
In addition, OP did offer to take her sister and niece with them. This did not meet with sisters approval because other children had been taken without their parents. Now, either this is a cover for her wanting her daughter taken away so she has an additional break, or it is an unrealistic expectation that her daughter can be treated exactly the same as other children in every circumstance- which is clearly not possible. Either way, she’s being unreasonable. I’m not saying I don’t have sympathy with her, especially in terms of being upset when she sees family interacting differently with her child than others in the family, but it doesn’t make her demands right or fair.
surely, it has to be unreasonable to demand your sister take your child with SN on holiday, when they have said they don’t think they can cope for a whole week because they’ve found her behaviour more difficult to manage this year than previously, if only because she is physically bigger and stronger? Why would you want to put your child or your sister in that position?