Hmm, maybe one of these diarrhea spells you get, you should not flush the toilet and let your sister find it. Then when she starts to complain, tell her that she's making it up because there's nothing wrong with your bowels, just like she said. You have perfectly nice poops, which you flush down the toilet, thank you very much.
Perhaps, when you are at the doctor's office, and your mom is in there with you, you need to open up to the doctor and tell him that people don't believe that you are sick, that your sister teases you, calls you a liar and makes you feel even worse, and does he know any way to educate your school, friends and even relatives about your condition and prove that it's real? That may make your mom realize that what your sister is saying is hurtful and might shame her into having a discussion with her and make her shut up. That way you can let your mom know you don't approve of what your sister is doing to you, but you're not confronting your mom directly or just sounding like you're whining. Your doctor may also tell her what she can do to help you put out the word that this is a very real and debilitating condition.