It’s not just about financials.

I don’t know why you get a better deal, but assuming it’s a favour to you from the landlord, then you’d be using up your good will (or owing favours back), for them. And 9k a year is a pretty big favour.

Even if they do make all their payments, if they are bad tenants in other ways you might lose any similar future favours (and maybe even cause your brother problems). So go with your gut.

And would the person you’re renting from know it’s not really you? Because again, you’d be increasing the risk of screwing up future favourable terms for yourself and/or your brother’s arrangement.

That being said, if all things considered the only problem is whether they will pay the rent on time, then tell them they can have it by paying 12 months up front, and the next 12 months up front in nine months time. That takes the (financial) risk off the table for you and saves them 9k a year, which they should recognise as owing you some pretty big favours when you need them.