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 :)
But you can say tembo mkubwa, which would mean big elephant, not older elephant. Why mix it up when you can just keep one adjective for elder and another adjective for big? Haha i get that it'd be a synonym for "big" too, just a lot of vocab to learn that's all! :) I love Swahili though... Asante Emilian!