Sorry you're having trouble with a non-supportive sister. Boecho has great advice...hopefully you can come to some agreement with her.
The diarrhea you're having is a result of the trauma of surgery. As boecho said, it's major surgery, and your upper GI tract has gone through a lot. What you're experiencing is called "dumping", and it's quite common. I had it on and off (it was streaky...I'd have it for a week, then it would subside, and later it would show up.) during my recovery. I still got it only very occasionally past the year mark, and figured I'd be dealing with it forever, when one day I realized it wasn't happening at all. Don't worry. It's not forever.
Here's a link to a diet that can be helpful in keeping dumping syndrome at bay: (It's very common after stomach and gallbladder surgery.)
Good luck with your recovery!
Words of wisdom by Eckhart Tolle:
"Whatever you fight, you strengthen, and what you resist, persists.”
“Worry pretends to be necessary but serves no useful purpose”
“Accept - then act. Whatever the present moment contains, accept it as if you had chosen it. Always work with it, not against it.”