My father died without a will

What is the name of your state (only U.S. law)? Missouri

My father passed away without a will. He married someone that we don't particularly know what well but that was his business.

He left behind a large farm and our stepmother is under the assumption that she holds the sole ownership to the estate and assets.

I looked up some MO law and I think that half of the estate belongs to my fathers children and I am looking for some guidence.

I am afraid that she is going to sell the place as soon as she can and leave the state.

What actions if any should I take to protect my intrests?

Thanks,

It depends upon how property was titled. If it was held jointly with right of survivorship or there were account beneficiary designations, then that property transfers outside of the probate process.

You might as well go on the offensive. Apply to the court to open probate and be appointed as the administrator. That, at least, gives you the legal authority to figure out who owns what and how it is owned and if the property is subject to probate and intestate succession.