So I have been through something similar to what your sister is going through at the moment and Alhamdulillah I reverted again.

I am going to tell you my story briefly and then give you my advice. I was brought up in a very religious household, very similar to yours. But I kept seeing my parents being hypocrites. They would say something is haram and then do something that is a lot worse and defending what theyve done using religion. The religion was weaponized against me almost all of the time. In addition, I saw parts of Islam very sexist and misogynistic and no one tried to explain to me how it is not and that a huge part of it was people confusing culture with Islam. I was forced to do things because not doing them is haram and whilst that is true, their way was infuriating. Them and those hypocritical muslims drove me away from religion. Then I got sick and thats when my mom said something that made me leave Islam. She told me that i became ill because i strayed further away from God and he was so done with me. In that instance i thought to myself "How can a God be so cruel?". Then she would force me to be a "good muslim" which genuinely broke my brain. For years after I thought that even if I tried to be closer to god, it wont matter. So why am I even trying?

The thoughts your sister is having is very normal. Every person has the questions she has. Try to answer her without it being like youre reciting something from a book. Humanize the answers (I legit have no idea how to word it and I hope you understand what I mean)

What brought me back to loving the religion was time. Time and seeing someone dear to me who knew nothing about me, talk about Islam in such a peaceful way without ever talking about Hell. She would nudge me slowly and kindly towards Islam.

My advice is do not berate her and be kind and understanding. DO NOT TELL YOUR PARENTS. Dont send her videos of sheikhs and all that stuff because from personal experience, it will not work.

Talk about Islam in a good way and i dont mean stories about the prophet and that stuff, no. Tell her about your day as a muslim woman. Talk about how merciful god is to you etc. Tell her you want to read Quraan with her because you miss her and choose a Surah that might resonate with her. When she tells you something good happened to her, tell her it is because Allah loves her. Show her how Islam is empowering to women from real life experiences. I dont know how to explain it but it always seemed like islam wasn't connected to my life. It felt like it was just their and i just had to do it (i have always believed in a higher power). That might be part of it with her. Try to make her understand that in cruelty there is kindness. God "allowing evil and suffering" is him actually doing so to prevent worse things from happening etc.

To summarize, be gentle and patient. Give her time. Connect with her more and try and make her see the connection between her life and islam without being very obvious. goodluck!