Yes - See an attorney to assist you with this
This is not intended to be legal advise or as legal representation. I am a California personal injury attorney . Be aware that every state has its own statute of limitations; and statutes & case laws that govern the handling of these matters.
The question lacks some specifics. The answer depends upon whom the check is made payable to-if it is made out to your brother, then he may be able to give you power of attorney to negotiate it. If the check was made payable to your mother's estate, then power of attorney will not work for you-an executor or administrator must negotiate the check.I recommend that you consult an attorney about this issue to discuss the facts in further detail.
Absolutely. An attempt to cash a check which is not written to you and without written authorization can have serious possible consequences. I suggest that you either take the check to him to sign or get a power of attoreny, a legal document giving you the right to endorse the check, from him. As a practical matter, it is probably easier and cheaper to take the check to him to sign if that is his desire.