What would you do if your girlfriend went shopping with her sister and you?

That depends on how old her sister is. If her sister is as old as her, don't be too ambiguous, such as joking with her (especially if they don't get along very well, your girlfriend will be jealous). If they are on good terms, it will be different. Everything depends on her happiness. But if she is young, it will be different. You can hug her (0~ 1 1 year). Buy her something to eat and make her happy. If she is (12~ 1 18 years old), then you can't get too close, because girls at this time are very ... well known, especially (13~ 17 years old). If you are too enthusiastic, she will not get used to it. So ...