<human>: Can you provide a brief rundown of the text in within 3 sentences?

I [24F] have been seeing my current boyfriend [27M] for a little over 6 months now. We met online and things have been going swimmingly to date. 

Given my parents 'doubts' about online dating I generally have not brought him around my family that much. This past weekend I decided to take him to my family's yearly barbecue as his first true extended family experience. 

I introduced him to my sister [20F] and I thought I could see a look of panic in her eye as I brought him up to introduce. I brushed it off, however. After the barbecue ended my sister came in to my room. There she laid out that about 2 years ago the two of them had hooked up for about 6 months. 

The two of them met on the bus on the way back from NYC. He commuted in to the city and she was just starting college in the city. She said the two of them struck a conversation which eventually led to dinner and then more. He, in fact, was her 'first'. It ended after about 6 months. Was never serious but more something physical and convenient. 

To his credit, later that night he called me and laid everything out on the table as well. No inconsistencies. I appreciate him being completely upfront about it.

Despite his honesty I have to say it freaks me out a little and since Saturday I have now been questioning if I can go forward knowing this. How do I work through?
<bot>: Been dating a guy for about 6 months. This past weekend learned he had a prior relationship (casualish) with my younger sister. Both were honest about it but it bothers me and not sure how I can move past it.