My experience/opinion from doing a lot of sail and canvas projects with a non "Sailrite" type of machine:
- A good home machine will be ok for your job provided you are very careful. Unless the Brother is from the 60's/70's, it is likely to have plastic worm gears inside. Too much stress will strip the gears. (As I did to my wife's Singer! Fortunately, internet mail order replacement parts enabled me to fix for only about $10.) (I subsequently found an older Kenmore that does only straight and zig-zag and is all metal inside -- my exclusive boat machine now.)
- Sacrificial fabric is relatively thin. And sailcloth that's been flying in the wind for a few years also can be poked easily with a needle. The most layers will be at the leach side. So your new fabric folded over + the leach tape + the field sail cloth = Five layers of cloth. But every 3-4 ft is an inch where the field sail cloth sections overlap = 6 layers. I think this will be ok.
- Don't even attempt to use the machine at the multiple and stiff layer head or at the clew or the tack. Even by "turning the wheel by hand". Won't work. The needle won't push through easily. Or even if you do manage to force it through, the thread itself can bind in the fabric which will cause a bird's nest of tangle underneath. Get out your punch awl and do these areas by hand.
- Set the machine to zig-zag. Don't do a straight stitch. Use widest from side-to-side and longest stitch length the machine will do. Check that the top and bottom thread tensions result in a good looking stitch.
- A home machine does not have enough foot pressure (nor a walking foot) to grip/feed the heavy/slippery cloth and also drag the weight of an entire sail consistently into the machine. If your wife is actually doing the sewing, you should continue to gather/pull/push/feed the sail gently into the machine as consistently as you can. Or she needs to do this if you are managing the machine.
- Thinnest sail maker's thread is best for a home machine. Needs to be UV resistant of course.
- Use the most robust needle you can find. At the local sewing store, this will be either for heavy denim. Or for leather.
- Before you invest in supplies, maybe first try sewing a few inches on your sail with some practice material to see how it goes for you. Afterward, the thread can be carefully cut without damaging the sail so the practice area can be removed.
- How are you going to pre-fasten the sacrificial fabric to the sail? Special double-sided tape is necessary. Don't use spray-on tack glue which turns yellow with UV exposure and age and becomes very noticeable.
The above of course assumes that you and/or wife already now how to use the machine and have maybe previously done some heavier duty household projects like upholstery covers for the garden furniture.
I never have had a problem with a home machine for things like restitching Sunbrella sail covers or my pedestal cover. But again, if its too many layers, like at the fastener fittings, "go hand".