Is it okay for my boyfriend to pick up a girl on his back at a concert, I didn't go there, and also he says its his sis?

He considers her as a sister, but i dont get good vibes from her, i told him to stay away but he said that she's always been a friend, so i let them go to the concert. So yesterday i called him after he came from concert and asked in general about picking me up at a concert when we go, so he fumbled and asked how do i know about picking up, so i was like what? He then told me that he picked that girl on his back at the concert, now i am furious and hurt, but he's on a trip and i dont wanna spoil it by asking all this.

Is it okay for my boyfriend to pick up a girl on his back at a concert, I didn't go there, and also he says its his sis?
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