Of course you can't help it, you've just had major surgery on your digestive system. I had these problems too for a while. Your digestive system has been modified, you've been on painkillers that also can have side effects impacting digestion, it'll take time for your body to adjust back to normal.
I'm sorry your sister isn't being very supportive. Have you explained to her what a major surgery you had? I'm guessing surgery is something she's never faced or had experience with. Maybe a sit down talk would help. Some people just react that way to things they don't understand. Maybe if she knew more about it, it would help her be more sympathetic. GERD is also one of those things people mistake with occasional heartburn. They may say, "you just have heartburn, what's the problem? That happens to everyone. Toughen up." But that's not how this works. None of us would have had surgery if it was that minor.
Its hard for others to understand this surgery or illness since you don't see any differences on the outside. Its not like a broken leg. You look normal on the outside, so why can't you just be normal? One of my closest friends got married shortly before my surgery. Before the wedding, I had skipped or left early from multiple social events due to severe pain from my GERD. At the wedding, I did my best to hold out as long as I could until I basically couldn't take it anymore, and had to leave early. I didn't hear from him for a month after that, but when I did, he basically unleashed on me about how inconsiderate I have been about cancelling/skipping plans, leaving the wedding early, and how I had just been a crummy, selfish person for months.
Turns out, I really hadn't done a good job of telling him (or any other friends) about how I had been feeling and what I was going through. Being sick was embarrassing and a private thing, so I held it inside. We sat down and talked for a long time. I told him about everything I had faced during my illness, about the surgery and how major it was, and how my recovery would be. We talked for over an hour. After I laid it all out, he was very understanding and even felt bad for how he had acted. I didn't hold it against him at all. Now he understands, and our friendship has been even stronger. He's actually going to be my best man in my wedding in December.
Your situation may be different, but there are similarities. She may just need to understand more in order to be sympathetic. Sit her down tonight, talk through it. Maybe it will help.