what do I do ugh

Okay so, I'm gonna tell yall something first. I am 14 going on 15 in a few months, and I have to share a room with my 3 year old sister. I am not allowed to keep her out of the room and I do not get any alone time unless I'm home alone. I have another sister, who has her own room even though she is younger than me. I've tried talking to my parents about putting a lock on my door or just giving me my own room but nothing has been done. I cant have privacy because no one knows how to knock and it's just extremely frustrating. (especially when I need some "me time")

Long story short, I don't know what to do and it's bothering me. I had my own room before but it got taken away because my sister complained until she got the room to her self.

Sorry for the long post, I just didn't know where else to go.