Rent to newly arrived sisters of good tenant?

[Johann Jells]

I have an application from 2 sisters, 27 & 23,   of a good tenant in a 3U for another unit in that building.  They have $85k income from decent jobs for a $1550 unit, so income isn't a problem. But they have no credit history and short work history in this country. Their brother has been a good tenant for a couple of years, and I could get him to cosign, he makes more than enough to pay both rents and not go over 30% of income (he's slumming a little). This is an educated, upper class, Latin American family.

So, should I insist on a co-sign or just go for it? If so , how do I write a co-sign lease? Just put him as the 3rd tenant?