I can see both sides.
Equal amounts of money would be fair in one way.
But I live down south, and my siblings live up north. I was looking for our first property at the same time as one of my siblings (who earns considerably more). We were looking at similar sized, 2-3 bed mid terrace. We talked about it quite a bit as we looked.
One time she said:
"Oh we looked round this glorious property. It was totally amazing, total mansion, but when we thought about it, it was ridiculously out of our price range-it was 120K".
To which I replied: "120K is more than the cheapest property round here!"
I don't think until that point she had quite appreciated the difference in prices. Where she was thinking way out of our price range, we were having to spend more.
If he said to both of you "I will give you a deposit" he may have assumed that it would be obvious to you that she would need more. All very well to say that she can sell at any time and use the money, but she probably won't because she needs a house near where she works just as much as you do.
If he'd given her 20k and it wouldn't cover a deposit, then to look at it another way, in London, I'd be pretty certain that money would have disappeared into the black hole known as rent money in 18 months maximum. Is that what your dad would have wanted?
To your dad, in this case, the fair thing was to give you both a deposit for a house where you are living. Unless you have reason to suppose that if you had lived in the opposite places he wouldn't have done that then you should be able to see his point.