I have a sister too but I think the better question to ask is why did you guys go out without her? was it something that she couldn't come along to? If its like going out for say a walk, run, or something which she could definitely join, she would understandably be upset for being left out I feel. It's better to reflect on the situation first so that the talk would be a more meaningful one. :)
Oh dear, I’m sure it’s just a misunderstanding. Perhaps find an opportunity to have a heart to heart talk with her? She’s your sis after all, I’m sure you’ll work it out.