OP, your children are never going to have an equal childhood. Your DD is always going to have had five years of undivided attention and five years of not having a sibling to play with. Your DS is always going to have had to share your attention when he was a baby, and always had a sister to play with (or fight with!)
Your DS gets more hand-me-down toys. Your DD gets more new toys - and has to hand down her old toys when she grows out of them, while your DS gets to sell them on Gumtree. They will have different relationships with their father, depending on your personal history. Your DD will always effortlessly win physical and mentally challenging games, because she's five years older. She'll also be expected to be more sensible and grown-up than her brother - because she's five years older. She'll be the one who has all the arguments with you about whether she's old enough to have a mobile or go to the shops on her own - by the time your DS came along, you'll have threshed all that out already. But your DS will probably get away with a bit more because he's younger - or your DD might think he does, even if he doesn't really.
At different times in their childhood, you'll have more or less money, space, time and energy, depending on your personal circumstances. They'll benefit or not benefit from that to different extents.
(And yes, I'm sure I got plenty of those things wrong, but I'm sure there are other differences I don't know about.)
As long as you love them both, and give them both your time and energy and affection, as long as you try and weigh up their differing needs and desires without consistently favouring one, they should turn out okay.
I think YABU. Your DD is an adult. She doesn't need pocket money.