Are you hoping to bypass her stomach? Do you think she's experiencing some form of gastroparesis? @Howard, might have some suggestions since that's what he has.
Also some of the feeding tube formulas like THIS one are also labeled for oral use. If you weren't trying to bypass her stomach you could possibly see if her doctor would write a prescription for one of those so she could trial it before having to go the harder route maybe. A peptide formula might be best because it's supposed to take some of the work out of digesting it.
Though if she has a lot of sensitivities even that might not work. I couldn't have that formula if I wanted it because of pea protein. When my Mom was in the hospital and they were doing TPS feedings, the dietician said that people who have issues with soy can also cross react to pea. ??
That's the other thing. Could she be sensitive to the foods she's eating. Even basic foods can cause issues depending on each individual. Can you try some other foods that she doesn't eat often. Gentle things like maybe avocado or sweet potato. IDK. Hopefully others will have some suggestions as well.
It's a really hard situation. You're a good brother looking out for your sis this way. Saying a prayer for both of you.