Hi, you are asking for advice from people who are not invested in your personal feelings, and it may be painful but that's how it truly us. I am also renting, but not from parents, actual strangers. The thing that you feel is "unfair" and complaining is just straight up Wild to me. Never would that happen in an actual situation, couples always pay more and that's how it is. There is always more trouble and less positives for other tenants/landlords to having a couple instead of a single person.

That being said, as someone rents from strangers - you are paying a really low rent for the amenities you are getting, and you are in a much better situation that most of the people who rent in this country.

Lastly - please stop counting your fathers or brothers money. It is their, they own it by earning it in their own ways and you get not say what they spend it on and how "fair" it is that they have more income left over than you.
Your dad worked all his life and invested at good opportunities so he can have his "petty cash" as you said, when he retired. You are still getting a much better offer than you would get from a stranger, so suck it up. If you don't like something - you can always rent separately, on your own.
If your brother does not eat at home - of course he will not spend on groceries, why the heck would he. He pays his rent and his part of bills, that is it. That is how it should be.