"is that no one owns the house?"

That's not what you told us. You told us there a 4 remaindermen (owners) and 2 life tenants (occupants), and that one of the LT died. That person no longer owned the home when they formed the Life Estate; they only owned the right to live there. And the remaindermen already own the property from that formation as well, which is why the death of a LT does not result in a step up in basis.

Don't confuse Possession and Ownership. Here is a nice explanation:

"The person holding the life estate -- the life tenant -- possesses the property during his or her life. The other owner -- the remainderman -- has a current ownership interest but cannot take possession until the death of the life estate holder."