Because it is not normal behaviour, it's massively, massively unreasonable and controlling behaviour, and it's not how decent men who respect their partners behave. I'm sorry.
Agree.
Why is it up to you to acquiesce or leave him?
Instead why not tell him how you intend to behave (or just do it) and let him react.
So tell your husband that you will be facilitating a normal relationship between your family and your children, including the brother/uncle he doesn't like. If your brother is in the UK, you will meet him and bring your children.
let him react to that. If he wants to break up a marriage because he doesn't want his children to meet their uncle whom he isn't fond of a few times a year, he should go right ahead. but if he does, during the time you have the children, you will be perfectly free to have them meet your brother.
This may be about control of you and his children. nip it in the bud right now. hope it isn't too late or an indication of controlling tendancies generally. Because if so, if you solve the brother problem, he will move on to something else.
Controlling men do often start with isolating women from their family and the reasons for minimising contact seem normal (he doesn't like me/she doesn't treat me with respect/if you loved me you wouldn't want to spend time with people who look at me like that).