Given the in-laws, I would go for the garage. OTOH, keeping temps and RH in the zone out there might be more difficult. Very hard to say without knowing what your environment will be like during your grow. Bottom line is that keeping your temps and RH under control is job 1 after providing light and nutes/medium. If that can't be done in the garage, you are left with the bathroom.
Keep in mind that even with your extraction fan and filter, there will be noticeable odor in the house at times, especially to someone entering the house for a visit. Even if your filter is perfect, if you open the zip to have a look or tend to plants, the lung room will get smelly for a while, and it will get into the rest of the house to some degree. If you are dealing with hostile in laws, your garage is your best bet, but you will have to sort out how to keep temps and RH in bounds, and keep the in-laws out of the garage.
I only have one issue with RH during my winter grow - keeping it high enough. I have used two methods, a small humidifier, and a container of water with a wet towel hanging into it and a fan directed at the towel. I prefer the towel method. If your issue is high humidity, your only option is a dehumidifier, and it might as well be in the lung room not the tent.
Keep the questions coming...