i'd be a bit annoyed at this and im afraid would have to ask why there was a difference made between the two, however, it is entirely up to your mother what she does with her money and she doesnt have to justify herself to anyone.
Maybe she thinks you might be forced to dip into it if you're not as financially comfortable as your brother. That's the sort of crappy judgement my mother makes on behalf of my ( wonderful) sister. It could easily drive a wedge between her, myself and my brother if we let it.
YANBU at all. If it was me I would need to know the reason why. But why is your brother asking this? Your nephew will have £280 in the account - I mean what does your brother mean "do" with it. Can your brother even get his hands on it? As others have said its your Mums money to do with as she pleases but its unfair (especially if this is to continue).
YANBU - it is entirely unfair when a parent favours one child over another and it is completely reasonable to feel hurt and a bit miffed by it. My mother favours my brother, which i find hurtful but i just get on with it now. (He was invited to my dad's surprise 60th birthday, i was not. When my mum won money to go on a 5 star holiday (£20000) she took my brother, his girlfriend, his best friend and his best friend's wife, my aunt and not me; when my dad was dying she sent me home from his death bed and told me she would call if anything happened and didn't phone until after he had died with her and my brother by his side.)