This would depend on what you are trying to convey. Kaka wangu mkubwa means "my brother is big" and Kaka mkubwa wangu means "the big brother is mine".
Kaka wangu ni mkubwa means "my brother is big" and Kaka mkubwa ni wangu means "the big brother is mine".
In your first example saying "Kaka wangu ni mkubwa" Then you are saying "My brother is big" Saying "Kaka mkubwa ni wangu" is saying "Big Brother is mine" which isn't correct in english or kiswahili. "Ni" basically is the swahili translation for the word "is" in english. The reason why its without "ni" is cause its saying Big brother swahili. Hopefully i helped :)
Does this translate to "older brother" or refer to the brother's physical attributes?