My thoughts....
I had to throw in my two cents and unless she experiences heavy psychosis during her manic phase I highly doubt that she is taking Haloperidol or a similar medication. If she is experiencing extreme psychosis there are much better antipsychotic medications (specifically atypical antipsychotic medications) that have less negative side effects than Haloperidol. They also have a decreased likelihood of causing extra pyramidal defects (i.e., loss of motor control). So unless she is on Haloperidol, I'm not sure that she is currently experiencing a loss of motor coordination.
If I was her therapist, I would simply ask something like this....
Has your sister in the past became upset and/or acted out in an aggressive manner while using computer devices?
The best predictor of future behavior is past behavior. If she has never had a problem using a computer, I wonder what makes you think that an Ipad could cause a problem. Originally, you asked if the Ipad is good for someone who is manic depressive, but I think your actual question is....Is a person with bipolar disorder likely to break an Ipad? If you are going to give the Ipad as a gift, you are not entitled to wonder how it will be used and if it will be broken in a week (think of christmas for a child). Oh, and why not just ask her what she thinks about the Ipad and whether or not she would be interested in owning one?
Being in mental health care field I appreciate your determination and devotion to your sister. I have worked with individuals diagnosed with bipolar disorder and I know the difficulty associated with this disorder and the trying times the disorder places on the individual and the family members.