I have always felt that any money parents or other relatives either give children (nieces, nephews etc) or leave in their wills should be evenly distributed.
However, a lot of people don't see it this way. My aunt left the bulk of her estate to my sister's children and only a small legacy to my sister and myself and nothing at all to her two other godchildren, even although she had said since we were children, that as she had no children, all four of her god-children would inherit equally. There was nothing we could do about it, but it did hurt, I admit. I felt very guilty towards the other two who were left out in the cold.
Presumably, your parents feel that your brother needs help to be able to put down the deposit for a house, and that as you already have a house, you don't neeed the same amount.
To my mind, this is only fair, if they helped you when you bought your house.
Whatever their reasoning, they should certainly not have lied to you and then handed over a smaller amount without any explanation.
So what do you do now?
If you ask your parents for an explanation, or even word it tactfully by saying, you had thought they had said they were going to give you both the same amount, you are likely to give offence, and, as you yourself said, feel that you sound ungrateful.
But you have already discussed this with your brother, which to me is unfair, as he presumably did not influence your parents, nor do I think you have any right at all to expect your brother to hand over some of what he received, either now or later.
However, as you and he have discussed the matter, it would be more honest if you or you both asked your parents why they first said even amounts and them apparently changed their minds.
Have you received sums from them in the past, when you brother received less, or nothing?
Please be careful here, as you may well stir up feelings that will hurt even more than your present hurt.
In your place, I would try just to shrug it off.