I totally agree with you since I also have a younger sister and my parents always ask me if its okay she does this or that. She also shares everything with me so I know what’s going on with her. My parents trust me to make the right decisions for her on their behalf becuase they know I’ve been through what she is going through now. The reason I think OP is not doing the right thing was because A) she shouldn’t snoop through her sisters phone etc (she should be able to talk to her) B) taking away computer/phone isn’t going to help and lastly C) embarrassing her infront of the family isn’t going to help. Sit her down and talk to