you were Sero's older sibling,and when she came over to hang out and listen to music with Sero,she of course met you.And it instantly clicked,you sometimes chilled with them,maybe even brought a few bottles of beer with you

And today,she came to hang out with only you,Sero was out of town with his parents so you two decided to get high together,even if you told her that it wasn't good for her.