My sister (15) will not stop lying and stealing

Its got to the point my parents have to lock their door, i have to lock mine

She will CASUALLY take stuff from my room then hold it up acting stupid to where I always find out

Shes taken candles from my room and swore, to my face, they were hers. Literally my own candles in front of me

Recently she acted all excited saying my bf bought me this phone (this bf is fake and a fake guys picture, everyone knows it but we pretend shes telling the truth to avoid drama). I said oh cool can I take a picture to see the camera quality (I knew it was stolen) she said sure, while I had it i looked through the camera roll and texts

It had pictures of a pregnant woman with texts from her friends and family on it! Like, over hundreds of photos of her with friends and family

I told her on the spot to return it. She said why? I told her to not play stupid and give back the stolen phone. I got sick of her saying no, it was hers

So i said im calling 911 now and telling them. She said go ahead ill delete everything (for once in her life actually being honest that she stole something)

I told my parents, my mom was absolutely enraged. She took the phone herself and is trying to return it to the woman in a way that won't trigger conflict. While she's fed up with my sister, she also wants to return it in a way where my sister didnt steal it

Shes never going to change. Shes lost friends. People all know her reputation of lying.

What should I do?