This is a complicated situation. You will need a good real estate attorney to open a probate estate in order to proceed. An executor will be appointed to handle the administrative tasks. At this point you will be able to contact the insurance company and ask them to cover the home during the probate process.
Locating your brother may involve placing ads or notices in newspapers or other publications. Hiring a private investigator is another option. If an exhaustive search doesn't locate him, the home may be able to be transferred by court order or another legal mechanism that wouldn't require him to sign off. This is rare though, so you do need an experienced real estate attorney to guide you through the process.
Once the probate process is complete and you are named as the heir, the insurance company should be informed of the change of status. You are free to insure with them again or compare rates with other insurance companies.
To sum it up, start the probate process as soon as possible. This will give you some legal basis to request the insurance renewal. If your mother and stepfather had a long term relationship with the insurance company, they may be happy to issue a policy and keep a customer.
It's very important to realize that legal requirements can vary from state to state. Please consult an attorney in your state.