I hope this message finds you well, present circumstances excluded. There are things you can do to protect assets that are justifiably yours. First however, let say I am sorry to hear about your loss. Things like death are tough enough to deal with in the absence of drama. I hope that settles down and you can grieve accordingly with peace in your life.
Freezing the accounts was a great initial move. Your next move should be to file for a restraining order to keep him away from the property until a formal probate of the estate. The executor of the estate can handle the restraining order through the court system, if in place. If not, you may have to do it yourself.
If your mother was the only beneficiary, and she was also his biological mother, then you will both be entitled to equal shares of the life insurance policy
(as will your sister), unless specified otherwise by some legal document, such as a will or trust. If the mother was the father's direct line beneficiary and she is not his biological mother, then he has no claim to the life insurance proceeds. It sounds like the former is the situation from your question above.
If there was no will, then the father is considered to have died intestate, which is a fancy legal term for dying without a will. In this event, the court will separate out how the estate is handled. As such, you need to make sure that this intestate succession is before the court as soon as reasonably possibly such that they can protect the assets from this brother until such time as disbursed.
In summary, if the brother is the biological son of the beneficiary (mother) he will be entitled to a portion of the life insurance, unless there is a document that specifies otherwise. As to the rest of the estate, you have taken the first major step in freezing the accounts. Get this estate before the courts as soon as possible and have a restraining order issued against the step brother to prevent pillaging of the possessions
that should be disbursed in legal fashion.
Let me know if you have any other questions or comments.
Best wishes going forward!