The original post: /r/amitheasshole by /u/SisterGroundedThrway on 2024-04-29 15:19:53.

I (25F) live in a three bedroom apartment with my fiancé (27M). We have a six month old son.

My younger sister Mia (fake name; 20F) recently started attending a university that’s closer to where I live than both my parents’ places. Because she needs to get to class early in the morning, my fiancé and I allowed her to move into our spare bedroom. She’s been living with us since January.

There’s a set of rules I’ve been having Mia follow since she moved in, mostly to make sure everyone can live peacefully. One of the most important rules concerns arriving after my son’s bedtime (usually between 19h and 20h). Mia is allowed to come home however late she wants, as long as she doesn’t make too much noise.

The door to the apartment has an electronic keypad lock, and the one we use also has a normal lock under it. Everyone has a copy of the key, but we all prefer typing in the password. However, the keypad is very noisy, and my son’s nursery isn’t far from the door. Every time the keypad is used after I’ve put the baby to sleep, he wakes up.

So naturally, the rule about getting home late includes not using the keypad. Everyone, including Mia, has known about this since day one.

At first, we had no problems. But now that Mia has gotten used to college life, she’s been going out at night frequently. This past month, she has arrived home after midnight every Friday and Saturday night. She always forgets the “don’t make too much noise” rule, and she has, on multiple occasions, used the keypad. My son wakes up crying every. Damn. Time.

I’d sit her down and remind her of the rule whenever this happened, but she’d continue to do it. My fiancé and I got tired of putting our baby back to sleep because of that, so I changed the password and told Mia she was no longer allowed to use the keypad. She agreed, and promised to try to make less noise.

On Friday night this week, Mia went out with her friends. She came home very drunk at 4AM, and forgot that I’d changed the password. Not only did she use the keypad, she also got the password wrong so many times that she triggered an alarm.

I was furious. Not only did my baby wake up crying, but I also got complaints from my neighbors.

The next morning, I told Mia that I’m establishing a curfew: while living with me, she needs to be home by 20h. She won’t be allowed to go out at night for the rest of the semester.

Mia put up a fight, saying I have no right to “ground” her like this or dictate what she does with her free time, but I held my ground. I told her she’ll be free to do as she pleases when she has her own place and raises her own family. While living with mine, she has to do as I say.

She told our parents about this. Our mother and stepfather are both on my side; our father is on Mia’s. He’s saying I’m not her mother, and I have no right to treat her like a child. He also thinks it’s unfair to do this to her over a drunken mistake.

AITA?