1) Modern cars may not support messing around with the battery. My BMW doesn't allow the user to change the battery. If I would do so, I would need to hook the car to the computer and use service software to activate the new battery! I guess that is not an issue for OP's current car, but he is better advised that most solutions suggested here, may not work in the future.
2) In my opinion, the best option is to just disconnect the battery after the car is parked in the garage. The car will consume battery power to keep the clock and other systems running even with the car switched off. Just disconnect the red (+) cable from the battery. This should allow the battery to keep charges for months.
3) If even longer periods are required, then it may be reasonable to take the battery out and regularily charge it in an aired environment - definitly not inside the house. For that a trickle char charger can be used. However, if the car is not moved for many months, the battery issue is just one problem! The tires will get deformed, oil impurities will cause sediment and eventually cloak tubes and random moving parts will glue themselves due to oxidation (wheels, breaks, maybe even motor components).
To maintain an unused car, it should be run every week or at least once a month.
I have been there and in the end I sold the car I inherited from my father, much to my own sadness. But I just did not have the time and resources to keep the car, which I did not use enough.
Another suggestion: depending on where OP lives, maybe renting a car for when it is needed, might be the cheaper and more reliable option? I understood that OP is not the driver, so why not have the driver come in a rented car? Sometimes the solution might be a bit outside the box - no offense meant.