My mum started paying my DS & DD spending money about 5 years ago every week. When my DD turned 18 recently she told her that she wouldn't be giving her any more spending but will still give my DS,who is 13, spending money. So overall when my DS reaches 18 he will have received more spending money than my DD. I think this is wrong because they should be treated the same. So because of this my sister has decided to give my DD spending money until my DS reaches 18 then they've both received the same, my mum doesn't know about this arrangement. I can't mention any of this issue to my mum as she's hard work to talk to about anything especially regarding money, life is easier to keep your mouth shut than to approach her about her decision.
AIBU with this situation?