I am a 23-year old college senior that temporarily returned from college during quarantine. Due to logistical necessity, since my brother moved into my former room when I left, I've been sleeping on the other side of my mom's master bedroom. Peculiarly, even though my sister has her own room, she has insisted sleeping in the same king-sized bed as my mother since she has been little. She is 18 years old now; I also have an 18-year old brother and a 21-year old brother who are staying here.
Inconvenient (and somewhat absurd) as this situation is, I now see firsthand that my sister has been causing a great deal of stress to my mother, especially when waking her up in the middle of the night, and I am trying to figure out how best to help deal with it. My mom leaves a sliding glass window open at night since otherwise it gets too hot and leaves her feeling unrested when she wakes up. My sister will come in and shut it before she goes to bed, usually about 3:00-5:00 a.m., and refuses to explain why. She also turns the bathroom light on to brush her teeth and floss typically ~1:30 a.m., often waking me or my mother up at that time. She often spends about an hour repeatedly blowing her nose and loudly gurgling water. If confronted, she starts yelling and often responds to behavioral critiques with insults, like telling my mother she wants to kill her or that she hates her. The other day, when my mother went to check on her in her room out of concern after my sister did not come to bed by 5:00 a.m., my sister pushed my mom into a lamp and gave my mom a large bruise on her hand. I am concerned that this level of violence could easily lead to a dangerous accident, especially since my mom is over 60 years old.
My mom works 6-7 days a week to buy food and other necessities, and typically wakes up at 5:30 a.m. When my sister wakes her up in the middle of the night, it often leaves her feeling tired, frustrated, and exhausted the next day. My sister refuses to acknowledge this, and typically responds that she is "tired too." It's disappointing that my sister relies on my mother's income for extracurricular activities, dental operations, and other expenses, but does not do her part in supporting my mother as she pushes herself to earn extra money.
So far, I tried setting a firm boundary where I would lock the door at a certain time the next day, if my sister closed the window the previous day without my mom's permission. I asked my two brothers for input, and we voted on it. However, after I followed through and let my sister in after she knocked, she closed the window anyway. I have tried placing large objects to block the window, but she moves them during the night. I even bought a motion sensor to try to dissuade her from trying to close it, but it only worked for visible light, not infrared. I recommended to my mom that we wear earplugs and refuse to open the door during the night, but my mom explained that she has already tried that, and expressed concern that my sister would physically break the door down.
My sister also makes other seemingly nonsensical demands, like insisting that the kitchen lights are turned off any time she is in the room, even if others are preparing dinner. My mom considered using her medical insurance to take my sister to the doctor to address these issues, but as she is 18 the doctor is not obligated to discuss the visit with my mother, and she is concerned that my sister will not acknowledge these issues in a 1-on-1 setting.
What are possible ways that I can help resolve this issue and protect my mother from abuse by my sister? She will be going to college in September and I will also likely be leaving, but both my mom and I need to wake up to do work in the morning over the next three months and we both want to sleep soundly.