Every once in a while I do work for my family and I always feel awkward asking for money.
My brother in law has written the paperwork to refinance our house and it was in the contract how much his percentage was. We signed it and the money came out of escrow.
He later knew how much we had in the bank and asked to borrow some money.
I had no problem charging him when he asked for my work.
When my nephew (child of same brother in law) needed graduation photos, I did this for free.
When my cousin asked me to photograph her wedding, I told her sure, and said we would have to meet to sign the agreement and to pick up the deposit. She never called me back, I called back once and since I never heard from her, I never shot it. I was removed from her FB friends list and her Uncle asked why I never showed.
When my niece asked me to photograph her wedding, I did so, but told her she had to pay my assistant because I couldn't do it by myself. I pulled the money off her veil after the money dance.
I get to decide who gets a discount or a goodwill job and no one else. Of course I could get screwed, but it won't happen twice.
I would ask your brother why he feels he deserves this discount. See what his reason was. Does he think the job was not done properly? Ask if everything is working properly and then wait for him to answer. It is business, family or not.