How do you get in-town siblings to help with their part instead of counting on the out of town siblings to do it all?

My problem is that although I live out of town and am on disability, I'm seen as the one to get my mother and stepdad out of the mess of 5 years of not filing their taxes. My stepbrother claims his work keeps him from gathering up 5 years of documents related to his dad's trailer park that he manages. He also claims that he does not have time to go to the house and get his dad's personal documents together. We've known about this problem since February and he's done nothing. Before my mother had her stroke in March, she and the helper at home gathered all of her tax stuff together. We are looking at a possible $400,000 due in back taxes plus penalties and interest not to mention the taxes that will be due from selling all of my mother's assests to pay this off. With her in the nursing home and her long term health insurance only covering so much,this could mean the house which is own jointly will have to be sold. However, none of this seems to bother my step-brother or my step dad. I have Durable POA over my mother but my step-brother has only some limited POA over his dad. This mess must be resolved soon as the IRS letters keep coming in.

Here's an artilce that might help:

https://www.agingcare.com/135062