My first response would be to check with your lawyer. But you say "the lawyer has not returned my calls."
I'm not a lawyer, but...
The lawyer, I would assume, reports to the executor of the estate. Are you one of the executors? If so, the lawyer should have a fiduciary duty to respond to you. If not, then such a duty may not exist. If, for instance, your brother or another relative is the executor, then contact the executor and explain your question and concerns.
If that avenue is unsuccessful, you may need to hire your own legal counsel to represent your interests.
A few other points: Despite your stating that the house "sold" in November and was "purchased" by investors, without a closing the house was neither sold nor purchased. What may exist is a ratified contract. Now, presuming that there is a ratified contract, the contract should specify a time frame for the transactions. Just as with any other sale, the investors would have x days to seek financing, and the property would close within y days. That should be in the contract, and that's what governs.
You ask if there's some kind of deadline that a bank has to close on a house. The bank isn't closing on the house. The buyers are. They may be obtaining financing from a bank. Or they may have their own sources of cash. Or they may be using private investors. The contract will specify the time frame that the buyers have to obtain financing.
If I had to guess, based solely on what you've provided, I'd guess that the investors have been unable to obtain financing. Your lawyer (and real estate agent) should be on top of the matter. If they aren't, it's time for you to get directly involved.
Again, not legal advice. Just an opinion.
Hope that helps.