Irene Yaymadjian, PsyD
It is commendable that you are concerned about your sister's coffee intake. Is she having any withdrawal symptoms, such as headaches, drowsiness, irritability, flu-like symptoms when not drinking coffee? Then yes, she may be addicted. You said "she ends up having" almost 3 to 7 cups of coffee. Is she "choosing" to drink coffee or does she have cravings for coffee? Is she drinking decaffeinated or regular?
In case she is addicted, then she needs to find out what are the underlying causes and deal with them. Some habits can be broken much more easily than others. She can start having every other cup of decaffeinated. She can also reach out for water instead of coffee. She can engage in deep breathing and relax herself. She can seek the help of a therapist who specializes in addiction.
Clearly you are concerned about your sister and rightfully so. Have you shared your concerns with her in a nonjudgmental way? I would ask her why she feels she needs so much. I would also ask about her sleep quantity and quality. She may drink coffee in order to compensate for inadequate sleep. Since caffeine at such high doses impairs sleep and its many important restorative functions, one problem may be compounding another.
I would recommend that your sister get an evaluation at a sleep disorders center; if she has sleep problems she may be amenable to that, and they will almost certainly discharge caffeine with her. If she agrees go with her. People typically underreport consumption of coffee, alcohol and other substances.
Finally, is it possible your sister is depressed and us using caffeine, a stimulant to boost her mood?
While you might discuss this people are generally more receptive to seeking treatment for sleep than mental health problems, but the sleep clinic work up will evaluate this as well.