How did you usually support your sister when something happened? I think she wants something in the same format. It seems to me that the best thing that can be done for a child is not to turn diabetes into a tragedy and, if possible, behave as before. From the outside, diabetes seems like something terrible, but in fact you get used to it quickly and the younger a person is, the easier it is to come to terms with diabetes.
There is one "secret" about being bullied by other children. People offend people in order to get a victim's reaction. It is impossible to offend a person by pointing out what (s)he is indifferent to. It doesn't matter if it's diabetes, appearance, clothes or something else. For example, imagine if you had very beautiful legs and you knew that they were beautiful and someone tried to laugh at their shape. You'd just look at this person as a fool, wouldn't you? She can say something like "What are you doing, aren't you going to pretend to faint yet? Go back and eat your pieces of sugar..."Do you dream of catching me when I faint? Oh, it is so romantic! Get in line." Do not make diabetes a topic that is not suitable for jokes. The calmer her family is about it, the calmer she is about her diabetes and the calmer her classmates are about it.
As a child, a person does not realize the danger of his illness, the age is too young to fully understand the meaning of the word "lifelong", and usually children consider themselves cool because they know how to give injections, this is something like an adult trait. My school friends were thrilled if I let them give me an injection (I don't recommend offering her friends something like that, I just want to show that children feel differently about it).
Praise your sister when she gives herself an injection, but don't show pity. And in no case do not let her manipulate the fact that you empathize with her. It is very convenient to manipulate with the help of illness, for example, to eat a couple of sugar cubes and skip school under the pretext of feeling unwell.
in fact, no one cares about someone else's disease, it's just that if a person is complex about it, it seems that everyone looks at you when you give an injection, but in fact people usually ask me once what I'm doing and why, and then they don't pay attention to it at all (both now and at school, and at the university, to be honest, people often don't notice it at all.)
You don't need to protect your sister from anything, you just need to make sure that she can handle her own problems, but at the same time know that you are always on her side and will always support her.