Being very limited on exercise I wouldn't feed her anything more than a vit/min supplement and maybe mix it in with some timothy pellets so she thinks she's eating.
I think even a ration balancer would be too much because it takes 1 - 2 pounds daily to meet the vit/min requirement.
With nothing but a vit/min supplement, it could be done in terms of ounces per day.
EquiPride takes ten ounces daily.
Tuttles Liquid 747 takes one ounce daily. I have my 25 yr old with EMS and hind gut ulcers on this. He is doing far better than I expected.
Or you could Google "equine vitamin mineral supplements" and you'll get a bunch of hits.
Less is more in her case.
Good luck to you and your mom and Kudos to your mom for trumping your brother
Sometimes moms have to step in, where it might be murky water and trouble if the sister tried