I don't want to sell my brother weed
Has your brother asked you to sell him weed?
Yes, but he is not a relaxed weed smoker and so I was against selling it to him.
It's ultimately your call and if selling weed to your brother is something that makes you uncomfortable then you shouldn't do it. Perhaps you're right and are just looking out for him.
I really don't want to sell it to him, plus we're no longer that close.
Siblings don't always maintain close relationships and you shouldn't feel pressured into doing something you don't want to. I feel confident that your brother can find another person to purchase weed from.