My sister has no remorse

My sister is 14 turning 15 and keeps stealing from me. Since she was young she has always stolen from me usually petty things like pens, books, jewelry, socks, food, you know the usual, and I've never thrown a fuss about it because it didn't matter to me too much. I'd notice things go missing, look for it, couldn't find it, and then forget it. And then months or years later I'll find it in her belongings. Usually I just go tell my parents and they'll scold my sister. I never understood why she would do it because my parents have always treated us equally and have always taken extra attention on her because she's always been a trouble kid. If she just asked my parents would buy it for her in less than a heartbeat.
I am 19 and the oldest of 4 with the youngest being 9, and my parents are in their 40s-50s. Usually parenting is left to me now a days because my parents are older, but I have discussed this issue with them and even they are at a loss at what to do. The reason I've posted this is because it seems to be escalating and we need some advice.
Three days ago she got into a fight with my parents refusing to do classes because my parents stuck the laptop on the desk so she can't go to her bedroom and shut the door and do it anymore. As a result she's been grounded and electronics taken away. The only thing she had anyway was a kindle and a laptop which must be used in public.
Everyone in my household has a kindle except for my dad, couple of weeks ago we found out that she had broken her own kindle and that's why she had been sharing my youngest sisters kindle. When my mum found out there were cracks all over it and smelled like spoiled milk. We weren't surprised because she never took care of it and always dropped it and drew my parents mad. Once it was found out my dad refuses to buy her a new one because its less than two years old and its expensive. I felt bad for her but it was her fault. She's never taken care of her things and definitely doesn't care about our things.
There has only been 2 instances where she has made me cry because of what she has stolen.
1- On holiday she stole a major clothing piece from me I had been using and refused to admit she took it even when we all knew she took it, especially when 3 hours after crying who do you think came out wearing it. My mum let me borrow hers but it just ruined all my outfits for the remaining week. She still has not admitted nor apologized. She was around 9-10 years old.
2- The most recent was two days ago my phone was stolen and so was my 9 year old's sisters kindle. I had a feeling she had taken it because i was with my family and she was in the bathroom and i called my phone and the call was declined and then the phone was powered off. Only one who could do it was my sister because she wasn't in the room and she was angry with us. I confronted her asking if she took it when she came out and she pleaded that it wasn't her and i believed her. I felt so crap that i asked her but it could have only been her. I thought i was wrong and i started looking and so did my mum. Couple minutes late my mum found it in the bathroom in a box where my sisters keep their dirty under wear until they wash it. I was heartbroken. Everyone knows that I am a germaphobe. I even take my own cup when i go out and i barely handle my own parents touches, and its just got worse once i became a medical student. I didn't care about the phone but rather where it was hidden. I could just imagine the germs crawling all over it and how i was even going to clean it. I can't just dunk the phone in bleach. Needless to say I burst into tears. I couldn't understand why she would do that to me. We aren't super close simply because we don't share any interests but I've always cared about her, and speak to her everyday freely.
At that time my sister was sitting on her desk and doing some homework, she didn't turn around, nor apologize, nothing. She just continued her work unbothered. I didn't know how to react other than cry. I've always know she was heartless, she's called me a *****, told me to kill myself when she was 12 but i forgave her with my parents insistence. She has no problem ruining other peoples things, she's threated to break my glasses, fought with my youngest sister and broken her water bottle, tried to break my youngest sisters kindle, etc. People have told us to destroy something precious of hers but we just don't have the heart to do it although she has no problem doing it. I just feel that if she goes low I shouldn't go lower. Me and my sisters have literally put our belonging under lock and key although me and my third sister have been doing it for longer, but I just feel soo bad for my youngest because she's heartbroken with her kindle missing. We were worried that she had broken it or something but we later found it hidden under some heavy furniture. My youngest just can't believe that her older sister could do that to her.
My sister has no remorse, she's recently made my mum cry and has pushed her around a bit before we got to her. My sister herself does not cry, we've tried grounding, taking things away even spanking but it just doesn't work. I'm worried that if she goes further no one will want to keep in contact with her.
Any Advice please, we are at our withs end.