My sister has panic disorder. Should I be worried about it?

Yes. Sure. Panic disorder can significantly reduce the quality of life for some individuals. Sometimes it leads to agoraphobia (fear of going outside in public), and sometimes evolves into bipolar disorder or recurrent depression. Some evidence suggests that panic disorder increases the risk for suicide. On the other hand, pure panic disorder is a very treatable condition using medication.
Please don't. The last thing your sister needs is someone else worrying about her "worrying". Panic disorder can be disabling, & it can be progressive. Encourage a calm & determined approach to her seeking effective help. This condition certainly can be overcome, but it requires ongoing professional support and therapy, in addition to personal responsibility for self-improvement -- & a calm, supportive family.