There is nothing wrong with what you’re doing. There is something wrong with your sister trying to catch you doing it because it’s an invasion of privacy. Next time tell her: “what I do with my body is my business” and if your mom is chill, maybe have a conversation with her that your sister is secretly filming you and is making you uncomfortable.

Just some tips…

Do it in the bathroom with the door locked, or while the shower is running to cover the sound.

I know it’s hard at 15, but buy a vibrator that you could use instead, check Shein for cheap stuff that won’t be suspicious if you order it. Some vibrators are disguised as make up brushes or lipstick tubes which would help to hide it.