that seems fair enough. I have the same situation with my brother. he's nearly 16 but is completely spoiled and mothered, and really is not expected to do anything. Don't give in! It's not going to help your brother or anyone. He should get off his arse and do it himself. And if your mum says 'don't be so pathetic, you're his older brother, help him out' (as mine would), just say he got himself into it, and whilst you care for him deeply, it's nothing to do with you. Also the tax people are going to find it really weird being called by a 17 year olds brother and probably wont be sympathetic.
Whatever happens, they should get over it. Not your fault