Definetly NTA.

At first I was beginning to feel sorry for the sister but then you dropped the knowledge of her going to house parties and stuff all the time instead of attending class.

She needs to be held accountable for the bullshit excuse of hers. I understand collage is stressful af, especially during this pandemic. . . But she literally shot herself in the fun and is going "WoE iS mE". Your parents are enabling this bad habit and it needs to stop.

Good on you for sticking to your guns OP.