Roasted Brussels - pre-roast and they’re soggy when I get there. Cook there and I take up the oven. Any other options?

Roast and put them in a salad, a la Momfuku recipe. Delicious. No softness. No need for hostess oven.

I often run them under the broiler as leftovers which wouldn’t take long when you get to her house.

Hi Stacy! Here's a "pot stuck" Brussels sprouts recipe that tastes great at any temperature: https://food52.com/recipes/24488-pot-stuck-brussels-sprouts

If you're open to switching to a raw Brussels sprouts recipe, that would travel really well. Here are a couple great options:

https://food52.com/recipes/76137-shaved-brussels-sprouts-with-brown-butter-vinaigrette-walnuts-pecorino
https://food52.com/recipes/39327-brussels-sprouts-caesar

Finally, you could follow whatever roasted Brussels sprouts recipe you were planning to use, leave them on the sheet tray (this way they're spread out, avoiding excess sogginess), then give them a quick toast under the broiler at your mother-in-law's. That wouldn't take too much time (and sometimes the broiler is even a separate space than the oven).

Hope this helps, and happy Thanksgiving!