How can a child sell their parent's assets in order to take care of medical expenses?

If you want to sell it while he's alive, you'd really need a power of attorney for real estate. Regardless of whether there's a will or not, the estate must pay any expenses prior to distribution, wuch as taxes, medical expenses, funeral expenses, administrative fees and other debts. Any distribution to heirs or beneficiaries would be from the remaining assets.

Is your stepbrother living in it for free, an owner, or is ther some legal arrangement? You'd want to make sure your grandfather is the sole owner of the house at this time for it to be part of his estate.

If there is no will inheritance would be dictated by the state's intestacy laws, which would also determine the shares going to you and your stepbrothers. The estate's personal represetnative would probate the estate and manage these issues, under whatever probate authority your state might have. You should contact an estate attorney in your area to suprvise this process and assist with probating the estate. There are many complex issues that you need to contend with and mistakes can be costly.