I wanted to buy my younger brother a phone on tab with his own line, however he isn't 18 yet, so I was wondering can I do the credit check with my details but have the line under his details? Or if there's any possibility I could get a line under my name and transfer it to him.
Your brother won't be able to have a line at all until he is of legal age. The only way you can get a line and phone for him is if you add it under your own (or parents) name and account. Of course he can use it but you (or they) will be the ones responsible for the usage and bills :)
When he turns 18, you can transfer the line to his own name of course.