Dog bit someone for the first time today?

Okay so me and my family have had our dog for 7-8 years now (shes about 9) and today she bit my sister for the first time. She's a very sweet dog, the most harm she has done is jumping on you when you walk through the front door (sometimes touching your hands with her teeth---shes never clenched down on her teeth to bit you), until today. Today my sister was petting her as usual and all of a sudden my dog jumped up and bit her three times in the face leaving her with three cuts, one above her eyebrow, one right below the eyebrow, and one underneath her eye. This has never ever happened before and it was a big shock to us. What should we do? She's not an aggressive dog, and right after she bit my sister she even immediately brought her ears back and tried to lick her face to almost fix it. :( We love our dog very much and are confused on why that happened and what to do about it. We don't want to lose her