Your contract likely contains a closing date. The seller(s) would have signed and agreed to that when you first came under contract.
Your agent should be working on this for you. They could send an email, with a document attached requesting that if the closing date is not met, through no fault of the buyer and due to the sellers lack of timely action, the seller agrees to cover the costs of extending your lock.
I can't guarantee this will work, but it is reasonable to request and may motivate a lackluster seller. With estate sales, people can become very unmotivated or are not wanting to do things which acknowledge the death of a loved one. Motivating them with the possible loss of the sale or another expense may move things along. In this time of easy faxing, scanning, e-signatures, it only takes a brief time to get something done if they want to.