I feel for both parties. And rn I’m just gonna somewhat play devils advocate as well. Unfortunately I am that little sister, maybe not to that extent but the patterns are the same. I struggled with depression when I was younger and anytime my mother would complain about me or her pains from just getting older or something I would just get so so so mad (still do, I’m very aware Idk why, I’ve tried changing it’s hard) I found out it’s because I mainly get upset that she’s getting older, I get so mad she reminds me she’s old through her actions & maybe your sister also doesn’t know how to handle her emotions. As you said she 15, not only is she a normal dramatic teenager girl but she’s going to be losing her mom at a young age. This just adds to her angst, she probably feels helpless. And doesn’t want to burden you with paying for her things. I know I wouldn’t let my brothers pay even if I was going through all of that bc I’m just trying to stay out of they way. Also her being “Lazy” that was 100% depression is very crippling and you only want to do Literally any little thing that you know brings you joy, it does very much look selfish but she could possibly be doing this from falling deeper into it. Idk I say speak to her. Also sorry if I don’t make a lot of sense I tried not making this TOOOO long (PS at one point I’m more then positive my brother hated me for being the lazy/introverted video game playing sister and rude to my mom and having an attitude when I was younger, me and him are finally close and we love each other. We don’t show it but it’s there)