You’ve been very generous with both your time and your money for storage. I think its time to put your foot down. As you yourself get older, you don’t want or need to leave this stuff there for someone else to deal with down the road. Use that as your excuse. I’d type up an email to your siblings and lay the facts out on the table. Tell them it is time to get rid of the storage facility and the items. Give them a firm date. They can either take possession of the items they want or split the costs of continuing to store it. Just be succinct and matter-of-fact about it. Let them know how much it is costing, etc. Ignore any possible backlash and feelings of guilt. Pat yourself on the back for dealing with it up to this point but move forward.