NTA
Maybe the parents offered the son and DIL a space in their house, rent free and most likely with no extra charges (bills, groceries) but this doesn't mean the sister has to cook more.
She's a kid, after all.
More than this, both brother and Sil are 37 years old. I think they are both capable of cooking too, but they feel entitled to be fed too.
The fact that SIL was always rude to the kid says all i need to know.
Btw: i cooked since 14 real meals for 10 people, at least 2 types of meals, every single day.
On holidays or whenever we had guests the number was way bigger.
Since i was cooking, i had to clean everything.
I had to make different meals not because of allergies, but because some didn't like it. So sometimes there were 4 types of meals every single day.
At least once a week they wanted cake or cookies.
At holidays i didn't get to eat until everyone finished because i was their server and only after i washed all the dishes and everything. Sometimes i was too tired to eat at all.
And i wasn't the oldest one. Only the 4th.
Now we're really LC or NC. I wasn't allowed to stop cooking.
So good on you, OP, because people don't have to be served.
They are able bodies and if they couldn't eat it, the solution isn't to treat your daughter as a servant.
Just cook for everyone (and don't forget to clean).
Btw: my husband knows the story and i cooked for him every day since the beggining. I do cook different for husband & 2 kids (likes and dislikes, sensory issues) but there's a difference here. They don't think it's my job to feed them.
They are grateful for the effort i make.
Keep your daughter close, OP. She's smart and she will always remember how you stood by her. I had nobody.