When my mum died three years ago I took Nobby, her beloved cat, in. He was a pesky little pest and I didn't really want him but I quickly grew to adore him. She was right all along: he was the happiest little soul.
When I took him I got a promise that when the time came, he'd be buried in my mum's garden alongside his brother and her other pets because that's what she would have wanted. He was her favourite. I thought it would be a long time.
On 2 January, being mischievous as always, he ran away from me and was crushed by a car in front of me. He died in my arms. There was so much blood. He loved going out with me by his side but I let him and my mum down that day because I didn't protect him and I will always feel guilty for that.
Circumstances meant that it wasn't to be that he got to rest in my mum's garden as she would have wanted. That upset and angered me: not for me, but for her.
But do you know what? I don't care now. He's been cremated and his ashes are in an urn in the shape of a curled up cat and he's with me. It's not good but it's good enough.
I tried to fulfill my mum's wishes, such as I thought they were, but it wasn't possible. So I did the best I could and my best is all she ever wanted.
However, I am finding it harder to take his death than hers. Weird, isn't it? He was just a cat. But he was the only thing I had left of her and now they are both gone.
So in my rambling way OP I think your husband should let his sister have a spoonful of their father's ashes to do what she wants with them. What's the harm? My mum's cat is just a cat. That's his sister's dad.
What I think is distasteful is not allowing her to remember him as she sees fit.