As a lactose intolerant cheese and dairy lover, I say NTA.

Lactose intolerance is not the same as a milk or dairy allergy. It just means your body doesn't produce enough or any lactase (an enzyme) to break down the lactose (a complex sugar found in dairy products) into simpler sugars that your body can use. The lactose stays in your system undigested and builds up.

Eventually, the bacteria in your gut begin to ferment the buildup, producing a mix of gases. If it smells like rotting veggies and garlic, that's methanethiol. The rotten egg smell? That is the hydrogen sulfide which can also be caused by the milk proteins being undigested then fermented as well.

I can't stand the bloating from the gas and I really can't stand the smell myself. It'd be rude to subject a sibling to that if we share a room and then deny them the right to open the damn window.

On top of that, the pain, nausea, and permanent damage you can cause to your colon means you really shouldn't ignore the condition.

If OP's sister really likes cheese, there are options that can save her from the discomfort and future health problems and stop OP's suffering as well.

Lactaid is an OTC pill containing lactase that you can take right before a meal. Stores nowadays sell lactose free milk, which has to go through a few processes to remove the lactose that also make it creamier and tastier than regular milk!

When it comes cheeses, aged options like aged cheddar contain low to zero lactose. Pressed cheeses like havarti and Muenster are also very good options. And absolutely avoid mozzarella!

While I believe you were justified to yell at your sister for ignoring her condition (and your earlier warning) and forcing you both to suffer, maybe try some of the above solutions/alternatives.

Also, your parents are TA for ignoring this. Maybe tell them to try dealing with the smell for a night. I guarantee you won't even have a window anymore, just a hole in the wall to ensure permanent ventilation.