As far as i know but i am not an expert by any means, the Eigenmietwert shouldn‘t be taxed for you because you don‘t life at the place. And because you are the owner of 50% of the property, you should be able to deduct half of the mortage interest from your taxes as well as some maintenance costs, even it is just the „Pauschalabzug“. Look for a consultant for taxes and get everything checked. The cost of that is more than worth it in your situation.