My sister physically locks her 3.5 y/o daughter in her bedroom from 7 pm to 7 am every night. Am I wrong in being concerned?

Title basically says it all. I am a dad to a toddler and feel sick knowing that my sister does this to my niece. They have turned the door knob around so the lock is in the hallway and my niece has no way to unlock the door. The excuse is that they’re afraid of her leaving her room and opening the front door but why not just teach your child or put a safety lock on the front door? Not only that, this seems like a major fire hazard. My sister has also made my parents do this to the room my kid and niece use when they spend the night for fear of them going down the stairs (I’m more sympathetic to this reason but that does not mean I’m ok with my child being locked in that room). So, my son won’t be staying the night at his grandparents house if the door knob is like that.

Am I being unreasonable? How do I address my concerns with both my sister and my mom or is it even my place? I’m legitimately concerned for my niece’s safety.

Edit: ok, my fire concern has been addressed. Thanks.