My dad died and left my sisters and I land. Will we be forced to sell during probate to pay his medical bills?

The estate is responsible for the bill. Now, that does not necessarily mean that the land must be sold. The hospital really doesn't care how the bill is paid, as long as its paid. So, you and your siblings can negotiate an agreement with the hospital to satisfy the bill in exchange for the hospital's agreement to cancel its claim against the estate.