iDiru
On that note, she eats Meowmix, because my mom is in charge of the food. But I have two other cats, who eat the same thing and don't have these problems.
And that is why she is shedding so badly. Meowmix, as you probably are aware, isn't a very good food.
Different animals react differently to their diet. You can't expect all cats to function the same on the same food. Why do you think some people get sick after consuming milk products but other people are fine with it? Some people can eat junk food and stay at a perfectly healthy weight, other people would become overweight if they ate junk food.
People and animals have individual needs. Your other cats may not be sensitive to grains and byproducts, but this cat is. You either can tailor this cat's food (or persuade your mom to) to something that will be much better on her body, or you can deal with the shedding.