In the 1901 census, George E Grant and wife Elizabeth, both born c1871, him in St Helier and her in St Martins, have one child: Clarence, born c1898 in St Helier.
In 1911 George is a carriage smith. George's widowed mother Mary, born c1840 in St Brelade, is with them in 1911, but Clarence is still the only child (and the couple are 40 years old).
For Donald to have been a brigadier in WWII, I think he would have been born by 1911 (born in 1911 would make someone 30 in 1941). Perhaps he was a brigadier only later, of course -- if he died in 2000, he was not likely born before 1911, or much before.
But I think it is very unlikely that this George E Grant is the father of her Donald Edward Grant, unless George and his wife had another child after 1911.
In 1891, Ethel Grant, born c1880 in St Helier, is in St Helier with brother George (as above) and mother Mary. In 1901, she is Ethel Hayward, married, living with mother Mary and brother Henry and sister Theresa. I don't see her in any census in 1911.
"Marie Thérèse" could be a combination of Ethel's mother's and sister's names, in French.
In 1881, the family in St Helier is
Mary J 41, wife, dressmaker
Matilda J, 12
George E, 10
Theresa A, 8
Maud M, 5
Henry LF, 3 (later called Harry)
Ethel B, 9 months
In 1871, Mary is there with children Charles, Matilda and George. She is a wife, dressmaker, but still no husband is present, unfortunately. (That is consistent with him being in the military perhaps.) There is no male Grant of the right age in Jersey to be her husband in 1871.
So yes, Ethel Blanche Grant Hayward 1880 and George E Grant c1871 were sister and brother.
But it seems unlikely that the Donald Edward Grant whom Marie Thérèse Hayward married was a child of Ethel Grant Hayward's brother George -- again, unless George had another child shortly after 1911, when his wife was 40.
Regarding Marie Thérèse Hayward, I checked this site
http://www.geopatronyme.com/which gives general information about birth surnames in France. Between 1891 and 1915 (as specific as it gets), there were 4 Hayward births there, and it breaks them down by département:
3 in Manche
1 in Paris
It is possible that between 1901 and 1915, Ethel Grant Hayward had one or more children in France. This could possibly account for the French given names of the child, which seem to be for Ethel's mother and sister, even though both parents had clearly English surnames, given the strict French naming rules.
You said first she was born c1913 and now you say c1915 -- but that was the middle of WWI and it might be unlikely that her mother would have been in Europe at that time. The earlier date seems more likely.
The only Alfred Francis Hayward born in England/Wales was born in 1877 in Cheltenham. so this would have to be Ethel's husband. His father was a bootmaker. I can't find him in the 1901 or 1911 census. I don't see him in army service records at findmypast, but if he had remained in the military until after the early 1920s, his records would not yet be released.
I searched in
http://www.london-gazette.co.uk/for the name Donald Edward Grant. If he was a brigadier in the UK, his name would absolutely appear there somewhere.
http://www.london-gazette.co.uk/issues/35034/supplements/125- promotion of Donald Edward Grant 159822 to 2nd lieutenant on 7 Dec 1940
(He was OCTU - Officer Cadet Training Unit (RAPC - Royal Army Pay Corps).)
http://www.london-gazette.co.uk/issues/35858/supplements/280- 1396962 Donald Edward Grant (133564) 12 Dec 1942
(I am not sure what this is a list of, but the heading seems to be "Ldg Acm", so I suspect this at least is the fellow who died in 1943.)
I didn't find anything for other versions of the name, e.g.
Grant, Donald Edward
Donald E Grant
Grant Donald E
There is a 2011 notice regarding an estate of a deceased Donald Edward Grant in Buckinghamshire, which I doubt would be him.
There might still be the possibility that Donald was the child of another brother of Ethel's. But I don't see him in any 1911 census, if so.
There is one interesting Donald Grant in 1911. His family is visiting a Dr MacGregor-Reid, born in India, in London. They are identified as visitors from South America. The father, Donald, is a Captain Mariner, born in the US. The wife was born in Brighton. Donald, aged 6, and his first sibling were born in the US, and the two younger children were born in Chile. They have a governess.
But then, there were Donald Grants all over Scotland too, and the rest of the world.
Hold on. Found Ethel in 1911. Overseas Military, India.
Ethel Blanche Hayward, born c1881, Jersey
3 children in her household:
Percival George Hayward, 7, born Ireland
Alfred John Hayward, 4, born Middleburg, Transvaal
Yvonne Ellen Hayward, 2, born Hong Kong, China
Husband Alfred must be there somewhere, but I'm not finding him.
