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Many people assume that the Grandmother was a dottering old fool who wasn't competent to execute a will that dictated her wishes. There's no evidence of that.
The LW was the one who characterized the will as "Grandma wanted the money split between her grandsons and her partners". Was that the language of the will? Probably not. The will has to specify exactly to whom and exactly how much.
Granny might have come to love the LW in her own right, maybe because G. knew that the grandson was not such a great person, LW was a saint for sticking with him and providing G. with the hope that she would reform the grandson. Who knows.
In any case: Grandma has made her wishes known. She wanted $10,000 to go to the LW. No further analysis or ponderings are necessary.
Leave your ageism and sexism behind. There's no reason to think she was senile, a fool or so blinded by sentimentality that she didn't know that relationships sometimes go south (i.e., women make bad decisions because they're overly emotional).
I find that older people, having been raised in a different day and age, have better manners and are more considerate than people from subsequent generations (e.g., they hold their tongues instead of calling people out). This seems to be interpreted as inscrutability, making the older person a blank canvas onto which people paint their own attitudes and desires, such as 'grandparents want their grandchildren/descendants to have all of their wealth'.
That's not necessarily true. Older people are complete individuals, with their own wishes of how to dispose of their wealth.
THE POINT: we know what Grandma wanted. She spelled it out in her will. She wasn't stupid or naive; to the contrary, she had been alive a long time and probably has seen it all.
She wanted you to have the money, LW. That's why she gave it to you. Keep it and use it for your own benefit.