You seem to be asking two different things - 1) can you change the successor executor in your mothers will, and 2) can you name a contingent beneficiary for your share of your mother's estate in the event of your death before your mother. In almost all cases, the answer to both questions is no, but I recommend that you consult with an attorney.
You can't change the terms of your Mom's will. If her will states, that if you die your brother will be her executor, then your brother will be her executor. The executor is the person who takes the will through court. If her will states, that if you die her estate will go to your brother, you can not change her will and give her estate to your husband. However, if you survive your Mom, and you inherit from her, then you control your inheritance, and you can leave it to anyone you want.
