Is my mother's will valid?

In Indiana, if your mother's will leaves everything to your step-father, and he is living when she passes away, your step-father will get everything, free and clear. He can then dispose of it as he pleases.

See a lawyer. You will need to show that your mother's will creates a trust, with you and your step-brother as remainder beneficiaries. This should be established sooner, rather than later.

No one has seen her will to know, but generally you cannot do what you said she did. (It is possible, with real estate, to do it, if worded correctly). You need to let a lawyer review the will.

It depends on the language of the will. You should consult with a lawyer. Unless the will puts the assets in trust or is a "contractual will", it is likely that your stepfather would not be bound by the will language about what happens to the assets after his death.

If the will has been admitted to Probate and a personal representative has been appointed to carry out the administration of the estate without challenge to the will or the capacity of your mother to make a will, then the will is valid. However, how the will reads and how it might be interpreted cannot be realistically commented on without seeing the actual document and understanding how it is being interpreted by either the personal representative or the court. It would be possible to draft a will that could accomplish what you have stated but the second part about it going to her children at the time of his death may be subject to interpretation depending on the wording. It probably means that your stepfather has a life estate in the property (meaning that he has the right to live there subject to the obligation to pay the taxes on the property and to not commit waste on the property - meaning that he would be obligated to maintain the property) with the remainder automatically going to you and your step-brother at his death or it could mean something else. I am assuming from your question that your adult stepbrother is the son of your stepfather and your mother. However, sometimes people might use the terms step brother or step sibling even if it is used to identify another child by your mother with a different father than yours. You should obtain a copy of the will and the probate documents from either your stepfather or from the probate court and have them reviewed by an attorney after an explanation of the relationships between the parties.