I know its easy to think landlords are evil, but in 99.9% of the cases they are not. Granted their are some "slum lords" out there but not many, the laws and regulations put on rental properties is very tight nation wide.
I retired very young {mid 30's} when I sold my business, I still own a lot of rental property {high mid to high end so by no means low income} and I treat my tenants very well, I have an office they can go with any issues 5 days a week 9-4. They have access to 24 hour building service, {I pay 2 contractors to be on call and if one of them can not make it I will personally go}. I also take care of all snow removal and landscaping. I include gas grills with every single unit so in a 3 family I have 3 gas grills installed, also video recording systems on all corners of the outside of the properties with wired alarm systems for each unit {I have had people not want to move in because of the cameras, lol, but figured it was for the best}.
Them are the good things, now the bad. Rent is due on 28th of every month and late after the 1st with a penalty {depending on the amount of rent and how late it is, it could be $500 if its late enough}. Condition is video documented and any damage including holes in walls for pictures costs money to fix {out of security deposit}.
I have had some angry tenants over the years, mostly because of security deposits, I recently had a woman have to pay over $900 of her security deposit because she painted a room {we had to repaint a neutral color she painted it lime green}, had a hole in a wall, a broken door handle {must have locked her key in, which all she had to do was call us and we would have opened it for her}, had a broken tile in a bath room, "lost" a mirror in the bathroom, etc... We actually got a lawyers letter demanding the entire deposit back, which we responded to with the 2 video inspections pre and post as well as invoices for the repairs and they never replied after that.
I go through every apartment between tenants, I have NO carpets {all tile and hardwoods}, all granite counter tops, paved driveways, etc etc etc so its basically a thorough cleaning, repaint where needed, fix any defects at all from light bulbs to discolored plumbing fixtures, new toilet seats, new shower curtains, new blinds {depending on age} etc... So its easy to see any damage after a tenant removes all their belongings...
ANYWAY, I would be surprised if your brother-in-laws landlord would raise his rent because he was getting a discount, if that is the case he should look elsewhere for shelter. A tenant landlord relationship should be strict and civil, I try my best not to get too involved in their business, I make it a point to meet with them at least once to introduce myself, then I let my office manager deal with everything else, I know I am way too easy on them and a softy, so if someone comes to me crying about money, I know I have been very fortunate in life so always help. I have one tenant that my office manager threatened to evict because I gave him 3 months free rent, he was in a custody battle for his kids and paying lawyers and I knew his exwife {not a nice person} so felt bad for him and told him not to worry about the rent until after the holidays, my office manager was NOT happy, {profits reflect on her bonuses}...
I would tell you bro in law to mention it to the landlord, if he is current and a good tenant I wouldnt think that signing a piece of paper would be an issue...